Long life sealed alkaline secondary batteries

ABSTRACT

In an aspect, provided is an alkaline rechargeable battery comprising: i) a battery container sealed against the release of gas up to at least a threshold gas pressure, ii) a volume of an aqueous alkaline electrolyte at least partially filling the container to an electrolyte level; iii) a positive electrode containing positive active material and at least partially submerged in the electrolyte; iv) an iron negative electrode at least partially submerged in the electrolyte, the iron negative electrode comprising iron active material; v) a separator at least partially submerged in the electrolyte provided between the positive electrode and the negative electrode; vi) an auxiliary oxygen gas recombination electrode electrically connected to the iron negative electrode by a first electronic component, ionically connected to the electrolyte by a first ionic pathway, and exposed to a gas headspace above the electrolyte level by a first gas pathway.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. ProvisionalPat. Application No. 62/611,946, filed Dec. 29, 2017, which is herebyincorporated by reference in its entirety to the extent not inconsistentherewith.

FIELD

This invention generally relates to alkaline batteries and in someembodiments more particularly to gas recombination methods and apparatusin sealed alkaline secondary batteries.

SUMMARY OF THE INVENTION

Provided herein are systems and methods for batteries and componentsthereof, for example, sealed batteries, such as sealed alkalinerechargeable batteries, providing enhanced performance including longerbattery life, increased discharge and cycling performance and enhancedbattery safety. In some embodiments, the battery systems and methodsutilize auxiliary electrodes for the recombination of gas generatedduring charging or discharging, for example, by chemical combustion ofstoichiometric quantities of oxygen and hydrogen, by electrochemicalreduction of oxygen gas and/or by electrochemical oxidation of hydrogengas.

Multiple auxiliary electrodes may be included in some embodiments forthe recombination of different gases generated during charging ordischarging of the electrochemical cell. The systems and methods areversatile and may be used with a variety of electrode, separator andelectrolyte components and compositions, including alkaline rechargeablebatteries, such as iron-containing batteries including nickel-ironbatteries.

In some embodiments, the systems and methods further include batteries,such as sealed alkaline rechargeable batteries, incorporating one ormore auxiliary electrodes that are capable of depolarizing the negativeor positive electrode, for example, in a manner that avoids, orminimizes, the loss of certain electrode additives, such as a sulfidespecies. In some embodiments, the systems and methods further includebatteries, such as sealed alkaline rechargeable batteries, characterizedby an arrangement of the positive electrode, negative electrode or bothprovided in uniform contact with a separator so as to eliminate, orminimize, spaces or structures in which gas bubbles may form, accumulateor travel, for example, in a manner to cause any gases formed toefficiently interact with the counterelectrode so as to encourage directrecombination, such as direct recombination of oxygen gas on thenegative electrode. In some embodiments, the systems and methods furtherinclude batteries, such as sealed alkaline rechargeable batteries,incorporating a positive electrode and/or negative electrode containinga hydrophobic polymer for enhancing transport of gases generated uponcharging or discharging the battery to a counter electrode or auxiliarygas recombination electrode.

In an aspect, provided is an alkaline rechargeable battery comprising:i) a battery container sealed against the release of gas up to at leasta threshold gas pressure, ii) a volume of an aqueous alkalineelectrolyte at least partially filling the container to an electrolytelevel; iii) a positive electrode containing positive active material andat least partially submerged in the electrolyte; iv) an iron negativeelectrode at least partially submerged in the electrolyte, the ironnegative electrode comprising iron active material; v) a separator atleast partially submerged in the electrolyte provided between thepositive electrode and the negative electrode; vi) an auxiliary oxygengas recombination electrode electrically connected to the iron negativeelectrode by a first electronic component, ionically connected to theelectrolyte by a first ionic pathway, and exposed to a gas headspaceabove the electrolyte level by a first gas pathway.

In some embodiments, the iron negative electrode further comprises oneor more sulfide compounds, the one or more sulfide compounds beingsoluble in the electrolyte. In some embodiments, the sulfide compoundcomprises one or more metal sulfides. In some embodiments, the one ormore metal sulfides comprise an iron sulfide, such as FeS, ZnS, CuS, MnSin powder or granular form added to the electrode, or iron sulfide(e.g., FeS, Fe₃S₄, Fe₂S₃ and/or other forms) generated in situ as areaction product or intermediary upon cycling the battery. In someembodiments, the one or more metal sulfides is iron sulfide. Inembodiments, the auxiliary oxygen gas recombination electrodeelectrochemically reduces oxygen gas in the gas headspace. Inembodiments, while electrochemically reducing oxygen gas in the gasheadspace the auxiliary oxygen gas recombination electrode depolarizesthe negative electrode potential and thereby prevents loss of thesulfide compound (e.g., one or more metal sulfides or a solid solutionincluding sulfide) by keeping the negative electrode potentialless-negative than a reduction potential of the metal sulfide.

Auxiliary electrodes may also be useful in combination with the positiveelectrode, for example, for recombination of hydrogen gas. Inembodiments, the alkaline rechargeable battery further comprises anauxiliary hydrogen gas recombination electrode electrically connected tothe positive electrode by a second electronic component ionicallyconnected to the electrolyte by a second ionic pathway, and exposed to agas headspace above the electrolyte level by a second gas pathway, whichmay be shared with the first gas pathway in some embodiments. In anembodiment, for example, the auxiliary hydrogen gas recombinationelectrode electrochemically oxidizes hydrogen gas in the gas headspace.In an embodiment, the system is configured to provide for a larger O₂recombiner than H₂. In embodiments, the auxiliary oxygen gasrecombination electrode has a greater specific activity than theauxiliary hydrogen gas recombination electrode, for example, where aratio of specific activity of the auxiliary oxygen gas recombinationelectrode to specific activity of the auxiliary hydrogen gasrecombination electrode is at least about 1.1, 1.2, 1.5, 2, 3, or more.

In embodiments, the second electronic component is configured to limitelectric potential of the auxiliary hydrogen gas recombination electrodeto a potential more negative than an oxygen evolution potential of theauxiliary hydrogen gas recombination electrode. In an embodiment, forexample, the second electronic component is a diode.

Various positions of the auxiliary electrodes will be advantageous incertain battery systems, which may depend on the compositions of theelectrodes and electrolyte, the amount of electrolyte relative theelectrodes, the geometry of the battery and other design factors. In anembodiment, the auxiliary hydrogen gas recombination electrode or theauxiliary oxygen gas recombination electrode floats on the electrolytesurface. In an embodiment, the auxiliary hydrogen gas recombinationelectrode or the auxiliary oxygen gas recombination electrode ispositioned in the battery container above a surface of the electrolyteand wherein the first or second ionic pathway comprises a wickingelement partially submerged in the electrolyte and extending to theauxiliary hydrogen gas recombination electrode or the auxiliary oxygengas recombination electrode.

In embodiments, the auxiliary oxygen gas recombination electrode isphysically separated from the negative electrode or the auxiliaryhydrogen gas recombination electrode is physically separated from thepositive electrode. In an embodiment, for example, the auxiliary oxygengas recombination electrode is at least partially submerged in theelectrolyte and the gas pathway comprises a hydrophobic element. In anembodiment, the positive electrode and the negative electrode are eachin uniform contact with the separator. In embodiments, for example, thepositive electrode, separator, and negative electrode are compressedagainst one another by a compressive pressure equal to or greater than athreshold compressive pressure. In embodiments, the thresholdcompressive pressure is at least 0.5 PSI, 1 PSI, or 2 PSI.

In embodiments, one of or both the negative electrode and the positiveelectrode contain a hydrophobic polymer binder, for example,poly-tetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP),or perfluoroalkoxy alkanes (PFA). In some embodiments, the PTFE, FEP, orPFA is fibrillated.

In an embodiment, for example, the negative electrode extends into theheadspace above the electrolyte level and the positive electrode isentirely submerged in the electrolyte. In an embodiment, for example,both the positive electrode and the negative electrode extend into theheadspace above the electrolyte level. In embodiments, the volume ofelectrolyte is equal to or greater than a cumulative total pore volumeof the positive electrode, the negative electrode, and the separator. Inembodiments, the auxiliary oxygen gas recombination electrode isentirely submerged in the electrolyte and the gas pathway is ahydrophobic element or a PTFE element.

In embodiments, the first electronic component is configured to limitelectric potential of the auxiliary oxygen gas recombination electrodeto a potential below a hydrogen evolution potential of the auxiliaryoxygen gas recombination electrode. In embodiments, for example, thefirst electronic component is a diode.

In an embodiment, the auxiliary hydrogen gas recombination electrode isphysically separated from the positive electrode. In an embodiment, theauxiliary hydrogen gas recombination electrode shares at least a portionof a substrate with the positive electrode. In an embodiment, thethreshold gas pressure is equal to or greater than 10 PSIG, 20 PSIG, 30PSIG, 40 PSIG, or 50 PSIG.

In an embodiment, for example, the auxiliary oxygen gas recombinationelectrode is also electrically connected to the positive electrode, andfurther comprising at least one switch configured to open and closecircuits between the oxygen gas recombination electrode and the positiveelectrode, and between the auxiliary oxygen gas recombination electrodeand the negative electrode.

In an aspect, provided is an alkaline rechargeable battery comprising:i) a battery container sealed against the release of gas up to at leasta threshold gas pressure, ii) a volume of an aqueous alkalineelectrolyte at least partially filling the container to an electrolytelevel; iii) a positive electrode containing positive active material andat least partially submerged in the electrolyte; iv) an iron negativeelectrode at least partially submerged in the electrolyte, the ironnegative electrode comprising iron active material; v) a separator atleast partially submerged in the electrolyte provided between thepositive electrode and the negative electrode; and vi) an auxiliaryhydrogen gas recombination electrode electrically connected to thepositive electrode by a first electronic component, ionically connectedto the electrolyte by a first ionic pathway, and exposed to a gasheadspace above the electrolyte level by a first gas pathway.

In an aspect, provided is an alkaline rechargeable battery comprising:i) a battery container sealed against the release of gas up to at leasta threshold gas pressure, ii) a volume of an aqueous alkalineelectrolyte at least partially filling the container to an electrolytelevel; iii) a positive electrode containing positive active material andat least partially submerged in the electrolyte; iv) an iron negativeelectrode at least partially submerged in the electrolyte, the ironnegative electrode comprising iron active material; v) a separator atleast partially submerged in the electrolyte provided between thepositive electrode and the negative electrode; and vi) an auxiliaryoxygen gas recombination electrode electrically connected to the ironnegative electrode by a first electronic component, ionically connectedto the electrolyte by a first ionic pathway, and exposed to a gasheadspace above the electrolyte level by a first gas pathway; whereinthe positive electrode and the negative electrode are in uniform contactwith one another through the separator.

In an embodiment, the positive electrode, separator, and negativeelectrode are compressed against one another by a compressive pressureequal to or greater than a threshold compression, for example, athreshold compression is selected over the range of 1 PSI and 10 PSI, 1PSI to 40 PSI, 10 PSI to 40 PSI, or greater than or equal to 40 PSI.

In an aspect provided is an alkaline rechargeable battery comprising: i)a battery container sealed against the release of gas up to at least athreshold gas pressure, ii) a volume of an aqueous alkaline electrolyteat least partially filling the container to an electrolyte level; iii) apositive electrode containing positive active material and at leastpartially submerged in the electrolyte; iv) an iron negative electrodeat least partially submerged in the electrolyte, the iron negativeelectrode comprising iron active material and a metal sulfide compound;v) a separator at least partially submerged in the electrolyte providedbetween the positive electrode and the negative electrode; and vi) asulfide reservoir containing a sulfide-source material operationallycoupled to the electrolyte for delivering a quantity of sulfide ions tothe electrolyte at a rate slower than a rate of dissolution of thesulfide-source material in the electrolyte. In some embodiments, thesulfide compound is soluble in the electrolyte. In some embodiments, thesulfide compound comprises one or more metal sulfides. In someembodiments, the one or more metal sulfides comprise an iron sulfide. Insome embodiments, the one or more metal sulfides is iron sulfide or asolid-solution comprising iron sulfide. The solid sulfide-sourcematerial may be soluble in said electrolyte and is characterized by itsdissociation into at least dissolved sulfide ions upon its dissolutionin said electrolyte.

A sulfide-source material being “operationally coupled” or“operationally connected” to an electrolyte refers to the sulfide-sourcematerial being connected to, exposed to, or in fluid communication withthe electrolyte such that the sulfide-source material may contributedissolved sulfide ions to the electrolyte via dissociation of thesulfide-source material. For example, an operational coupling between asulfide-source material and the electrolyte may be constant and/orintermittent. Similarly, a counter ion-source material being“operationally coupled” or “operationally connected” to an electrolyterefers to the counter ion-source material being connected to, exposedto, or in fluid communication with the electrolyte such that the counterion-source material may contribute dissolved counter ions to theelectrolyte via dissociation of the counter ion-source material. Forexample, an operational coupling between a counter ion-source materialand the electrolyte may be constant and/or intermittent. Dissolution ordissociation of a material, such as a sulfide-source material, refers todissolution or dissociation, respectively, of any fraction or portion ofthe material. The terms “operationally coupled” and “operationallyconnected” may be used interchangeably.

In an embodiment, the alkaline rechargeable battery further comprises anauxiliary oxygen gas recombination electrode electrically connected tothe iron negative electrode by a first electronic component, ionicallyconnected to the electrolyte by a first ionic pathway, and exposed to agas headspace above the electrolyte level by a first gas pathway,wherein the auxiliary oxygen gas recombination electrodeelectrochemically reduces oxygen gas in the gas headspace, and whileelectrochemically reducing oxygen gas in the gas headspace depolarizesthe negative electrode potential and thereby prevents loss of thesulfide compound. In embodiments, the auxiliary oxygen gas recombinationelectrode comprises a sulfide compound (such as a metal sulfide or asolid solution comprising a metal sulfide) that is soluble in theelectrolyte and is positioned at least partially in the headspace abovethe electrolyte level.

Encapsulation and/or shield materials may be included to provide acontrolled release of the sulfide reservoir to ensure that sulfide isavailable within the reservoir for a longer period of time. Inembodiments, for example, the sulfide reservoir comprises sulfide-sourcematerial encapsulated in an encapsulation material that dissolves in theelectrolyte at a slower rate than the sulfide-source material. In anembodiment, the encapsulation material comprises a metal oxide, forexample, one or more of zinc oxide, aluminum oxide, bismuth oxide, andcopper oxide. In embodiments, the encapsulation material comprises oneor more of silicon oxide, silicon dioxide, a polymer, polyvinyl alcohol,and polyethylene.

In an embodiment, the sulfide reservoir comprises a sulfide-sourcematerial contained within a shield material that is entirely insolublein the electrolyte. In embodiments, the shield material comprisesstainless steel or a polymer impervious to the electrolyte. Inembodiments, the sulfide reservoir comprises a sulfide-source materialsealed within a bag made of an anion exchange membrane submerged in theelectrolyte.

In some embodiments, the sulfide reservoir is submerged in theelectrolyte. In some embodiments, the sulfide reservoir is positioned inthe headspace and joined to the electrolyte by an ionic pathway. In someembodiments, the sulfide reservoir is placed directly within thenegative electrode. In some embodiments, the sulfide reservoir iselectrically connected to the positive electrode by anintermittently-openable electrical connection. In embodiments, forexample, the intermittently-openable electrical connection comprises aswitch operated by a controller, a temperature-sensitive switch, anelectronic oscillator or any combination thereof.

In some embodiments, the battery further comprises a closed-loopautomatic control system; wherein said control system comprises asulfide detector configured to detect an event or condition; and whereinsaid control system comprises an actuator configured to deliver orexpose said sulfide-source material to said electrolyte upon detectionof said event or condition by said sulfide detector. In someembodiments, the event or condition is selected from the groupconsisting of a sulfide ion concentration, a battery performance metricthreshold or change thereof, a temperature threshold, or any combinationthereof.

In an aspect, provided is an alkaline rechargeable battery comprising:i) a battery container sealed against the release of gas up to at leasta threshold gas pressure, ii) a volume of an aqueous alkalineelectrolyte at least partially filling the container to an electrolytelevel; iii) a positive electrode containing positive active material andat least partially submerged in the electrolyte; iv) an iron negativeelectrode at least partially submerged in the electrolyte, the ironnegative electrode comprising iron active material and a sub-oxide of asulfide compound; and v) a separator at least partially submerged in theelectrolyte provided between the positive electrode and the negativeelectrode. In some embodiments, the sulfide compound comprises one ormore metal sulfides. In some embodiments, the one or more metal sulfidescomprise an iron sulfide. In some embodiments, the one or more metalsulfides is iron sulfide or a solid solution comprising an iron sulfide.

In an aspect, provided is a method of operating a battery to maintain asulfide compound in a negative iron electrode, comprising: i) chargingand discharging a battery comprising a positive electrode and an ironnegative electrode containing a sulfide compound; and ii) preventing thenegative electrode from ever being charged to a state-of-charge greaterthan 50%, 60 %, or 70%. In some embodiments, the sulfide compoundcomprises one or more metal sulfides. In some embodiments, the one ormore metal sulfides comprise an iron sulfide. In some embodiments, theone or more metal sulfides is iron sulfide or a solid solutioncomprising an iron sulfide.

In embodiments, for example, decreasing the voltage applied to thenegative electrode comprises making the negative electrode half-cellpotential less negative than -1.5 V, -1.2 V, -1.15 V, or -1.1 V vs. aHg/HgO (mercury/mercury oxide, or “MMO”) reference.

In an aspect, provided is a battery comprising a positive electrodecontaining positive electrode active material and a negative electrodecomprising an iron active material and a sulfide compound, wherein atleast 5%, 10%, or 20% of the iron active material is an iron hydroxidefor all states of charge of the battery. In some embodiments, thesulfide compound comprises one or more metal sulfides. In someembodiments, the one or more metal sulfides comprise an iron sulfide. Insome embodiments, the one or more metal sulfides is iron sulfide or asolid solution comprising an iron sulfide.

In an aspect, provided is a metal-iron battery comprising a positiveelectrode and a negative electrode comprising an iron active material,wherein at least 5%, 10%, or 20% of the iron active material is ironhydroxide when the positive electrode is fully charged.

In an aspect, provided is a metal-iron battery comprising: i) a positivemetal electrode comprising positive electrode active material; ii) anegative electrode comprising iron active material and a sulfidecompound; iii) the positive and negative electrodes contained within ahousing sealed to prevent escape of gasses from the housing; and iv) agas recombination device connected to the positive electrode. In someembodiments, the sulfide compound comprises one or more metal sulfides.In some embodiments, the one or more metal sulfides comprise an ironsulfide. In some embodiments, the one or more metal sulfides is ironsulfide or a solid solution comprising an iron sulfide.

In embodiments, for example, at least 5%, 10%, or 20% of the iron activematerial is iron hydroxide at all states of charge. In an embodiment,the battery further comprises a sulfide reservoir. In an embodiment, thesulfide reservoir comprises a sulfide-source material, the sulfidereservoir delivering a quantity of sulfide ions to the electrolyte at arate slower than a rate of dissolution of the sulfide-source material inthe electrolyte.

In an aspect, provided is a battery comprising: i) a positive electrodehaving a quantity of positive active material defining a positiveelectrode capacity; ii) a negative electrode having a quantity of ironactive material defining a negative electrode capacity that is at least10% greater than the positive electrode capacity; iii) the positive andnegative electrodes contained within a housing sealed to prevent escapeof gasses from the housing; and iv) a gas recombination device withinthe battery container and electrically connected to at least one of thepositive electrode and the negative electrode.

In embodiments, for example, the negative electrode capacity is at least1.5 times, at least 2 times, or at least 2.5 times greater than thepositive electrode capacity. In an embodiment, the negative capacity andthe positive capacity are defined as end-of-formation capacity. In anembodiment, the negative capacity and the positive capacity are definedas rated capacity. In an embodiment, the negative capacity and thepositive capacity are defined as theoretical capacity.

In an aspect, provided is a battery comprising a positive electrodecomprising a positive electrode active material and a negative electrodecomprising an iron active material and a sulfide compound, wherein atleast 5% of the iron active material is in a discharged state at allstates of charge of the positive electrode. In some embodiments, thesulfide compound comprises one or more metal sulfides. In someembodiments, the one or more metal sulfides comprise an iron sulfide. Insome embodiments, the one or more metal sulfides is iron sulfide or asolid solution comprising an iron sulfide.

In some embodiments, the battery comprises a solid sulfide-sourcematerial operationally connected to the electrolyte. In someembodiments, the solid sulfide-source material is soluble in saidelectrolyte and is characterized by its dissociation into at leastdissolved sulfide ions and dissolved counter-ions upon its dissolutionin said electrolyte. In some embodiments, a total concentration ofdissolved counter-ions in the electrolyte is greater than aconcentration of said dissolved counter-ions derived from dissolution ofsaid solid sulfide-source material. In some embodiments, a ratio oftotal concentration of said dissolved counter-ions in the electrolyte toconcentration of said dissolved sulfide ions in the electrolyte isgreater than the stoichiometric ratio of counter-ions to sulfide ions insaid solid sulfide-source material. In some embodiments, the batteryfurther comprises a solid counter-ion source material exposed to theelectrolyte and configured to limit a concentration of said dissolvedsulfide ions in the electrolyte to less than or equal to a thresholdconcentration; said threshold concentration being less than a solubilitylimit of the solid sulfide source material in an identical electrolytefree from exposure to said solid counter-ion source material; said solidcounter-ion source material being soluble in said electrolyte and beingcharacterized by its dissociation into at least said dissolvedcounter-ions upon its dissolution in said electrolyte. In someembodiments, a concentration of dissolved sulfide ions in saidelectrolyte is less than or equal to a threshold concentration; saidthreshold concentration being less than a solubility limit of the solidsulfide source material in the electrolyte at a temperaturecorresponding to the temperature of the electrolyte. In someembodiments, the iron negative electrode comprises the solidsulfide-source material. In some embodiments, a concentration of sulfideions in the electrolyte is greater than zero and less than 700 µM. Insome embodiments, a ratio of total moles of dissolved and undissolvedcounter-ions in the battery to total moles of dissolved and undissolvedsulfur species (i.e., sulfide-source materials, sulfide ions, sulfites,and sulfates) in the battery is greater than the stoichiometric ratio ofcounter-ions to sulfide ions in said solid sulfide-source material. Insome embodiments, the solid sulfide-source material is a metal sulfideother than iron sulfide. In some embodiments, the counter-ion is a metalion other than iron. In some embodiments, the battery comprises anexcess concentration of counter-ions in the electrolyte sufficient todepress the concentration of dissolved sulfide ions below a solubilitylimit of the solid sulfide-source material in an otherwise identicalelectrolyte without the excess concentration of counter-ions. In someembodiments, the electrolyte contains dissolved counter-ions capable offorming a sulfide compound with dissolved sulfide ions; and wherein thecounter-ions are present in a concentration sufficient to depress asolubility of the solid sulfide-source material below its solubilitylimit in a counter-ion free electrolyte otherwise identical to theelectrolyte at a temperature corresponding to the temperature of theelectrolyte.

Generally, when a concentration of a first species in an electrolyte iscompared to a solubility limit of the first species or of a secondspecies in the electrolyte, the value of the solubility limitcorresponds to that in an electrolyte having a temperature that issubstantially equivalent to the electrolyte temperature in which thefirst species is dissolved. For example, if a concentration of a firstspecies in an electrolyte is said to be 10% less than its solubilitylimit in the electrolyte, then the concentration of the first species inthe electrolyte having an instantaneous or average temperature of X °Cis 10% less than its solubility limit in the electrolyte having aninstantaneous or average temperature of the same X °C, where is a Xnumerical value corresponding to a temperature.

In an aspect, a battery comprises: a battery container; a liquidelectrolyte at least partially filling the container to an electrolytelevel; a positive electrode containing positive active material and atleast partially submerged in the electrolyte; an iron negative electrodeat least partially submerged in the electrolyte, the iron negativeelectrode comprising an iron active material and a solid sulfidecompound that is soluble in the electrolyte; wherein a concentration ofsulfide ions in the electrolyte is greater than zero and less than 700µM.

In an aspect, a battery comprises: a battery container; a liquidelectrolyte at least partially filling the container to an electrolytelevel; a positive electrode containing positive active material and atleast partially submerged in the electrolyte; an iron negative electrodeat least partially submerged in the electrolyte, the iron negativeelectrode comprising an iron active material; and a solid sulfide-sourcematerial operationally connected to the electrolyte; the solidsulfide-source material being soluble in said electrolyte and beingcharacterized by its dissociation into at least dissolved sulfide ionsand dissolved counter-ions upon its dissolution in said electrolyte;wherein a ratio of total moles of dissolved and undissolved counter-ionsin the battery to total moles of dissolved and undissolved sulfurspecies in the battery is greater than the stoichiometric ratio ofcounter-ions to sulfide ions in said solid sulfide-source material, andin some embodiments wherein the counter-ion is a metal ion other thaniron.

In an aspect, a battery comprises: a battery container; a liquidelectrolyte at least partially filling the container to an electrolytelevel; a positive electrode containing positive active material and atleast partially submerged in the electrolyte; an iron negative electrodeat least partially submerged in the electrolyte, the iron negativeelectrode comprising an iron active material; a solid sulfide-sourcematerial operationally connected to the electrolyte; the solidsulfide-source material being soluble in said electrolyte and beingcharacterized by its dissociation into at least dissolved sulfide ionsand dissolved counter-ions upon its dissolution in said electrolyte; andan excess concentration of counter-ions in the electrolyte sufficient todepress the concentration of dissolved sulfide ions below a solubilitylimit of the sulfide-source material in an otherwise identicalelectrolyte without the excess concentration of counter-ions. In someembodiments, the excess concentration of counter-ions corresponds to aconcentration greater than a concentration of said dissolvedcounter-ions derived only from dissolution of said solid sulfide-sourcematerial. In some embodiments, the electrolyte comprises an excessconcentration of counter-ions prior to a first charge-discharge cycle.In some embodiments, the battery is an alkaline rechargeable battery andwherein the electrolyte is an aqueous alkaline electrolyte.

In an aspect, a battery comprises: a battery container; a liquidelectrolyte at least partially filling the container to an electrolytelevel; a positive electrode containing positive active material and atleast partially submerged in the electrolyte; an iron negative electrodeat least partially submerged in the electrolyte, the iron negativeelectrode comprising an iron active material; and a solid sulfide-sourcematerial operationally connected to the electrolyte; the solidsulfide-source material being soluble in said electrolyte; theelectrolyte containing dissolved counter-ions capable of forming asulfide compound with dissolved sulfide ions; and wherein thecounter-ions are present in a concentration sufficient to depress asolubility of the solid sulfide-source material below its solubilitylimit in a counter-ion free electrolyte otherwise identical to theelectrolyte at a temperature corresponding to the temperature of theelectrolyte.

In any embodiment, the iron negative electrode may comprise the solidsulfide-source material. In some embodiments, the iron electrodecomprises an incorporated sulfide compound formed during formationcycling. In some embodiments, prior to a first charge-discharge cycle,the iron electrode is free of a sulfide compound. In some embodiments,the iron electrode further comprises a sulfide-additive prior to a firstcharge-discharge cycle. In some embodiments, the battery comprises asulfide reservoir prior to and/or after a first charge-discharge cycle.In some embodiments, a counter-ion solution is added to the electrolyteafter a first charge-discharge cycle. In some embodiments, a counter-ionsolution is added to the electrolyte before a first charge-dischargecycle.

In some embodiments, the battery comprises a solid counter-ion sourcematerial exposed to the electrolyte; said solid counter-ion sourcematerial being soluble in said electrolyte and being characterized byits dissociation into at least said dissolved counter-ions upon itsdissolution in said electrolyte.

In some embodiments, the total concentration of dissolved counter-ionsin the electrolyte is at least 10% greater, at least 20%, at least 30%,at least 40%, at least 50%, at least 60%, at least 70%, at least 100%,at least 150%, at least 200%, at least 400 %, at least 500%, at least700%, at least 900%, or for some applications at least 1,000% or moregreater than a concentration of dissolved sulfide ions in theelectrolyte. In some embodiments, the total concentration of dissolvedcounter-ions in the electrolyte is at least 10% greater, at least 20%,at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, atleast 100%, at least 150%, at least 200%, at least 400%, at least 500%,at least 700%, at least 900%, or for some applications at least 1,000%or more greater than a concentration of dissolved counter-ions derivedfrom dissolution of said solid sulfide-source material during at least1% of the charge-discharge cycles throughout the lifetime of saidbattery.

In some embodiments, the total concentration of dissolved counter-ionsin the electrolyte is at least 1%, at least 5%, at least 10%, at least20%, at least 30%, at least 40%, at least 50%, at least 60%, at least70%, at least 100%, at least 150%, at least 200%, at least 400 %, atleast 500%, at least 700%, at least 900%, or for some applications atleast 1,000% or more greater than a concentration of dissolvedcounter-ions derived from dissolution of said solid sulfide-sourcematerial during at least 1%, at least 2%, at least 5%, at least 10%, atleast 20%, at least 50%, at least 75%, at least 80%, at least 90%, atleast 95%, or preferably for some applications at least 99% of thecharge-discharge cycles throughout the lifetime of said battery. In someembodiments, the ratio of total concentration of dissolved counter-ionsin the electrolyte to concentration of dissolved sulfide ions in theelectrolyte is at least 10% greater than the stoichiometric ratio ofcounter-ions to sulfide ions in said solid sulfide-source materialduring at least 1% of the charge-discharge cycles throughout thelifetime of said battery. In some embodiments, the ratio of totalconcentration of dissolved counter-ions in the electrolyte toconcentration of dissolved sulfide ions in the electrolyte is at least1%, at least 5%, at least 10%, at least 20%, at least 30%, at least 40%,at least 50%, at least 60%, at least 70%, at least 100%, at least 150%,at least 200%, or preferably for some applications at least 210% greaterthan the stoichiometric ratio of counter-ions to sulfide ions in saidsolid sulfide-source material during at least 1%, at least 2%, at least5%, at least 10%, at least 20%, at least 50%, at least 75%, at least80%, at least 90%, at least 95%, or preferably for some applications atleast 99% of the charge-discharge cycles throughout the lifetime of saidbattery.

In some embodiments, the threshold concentration is at least 10% lessthan the solubility limit of the sulfide-source material in an identicalelectrolyte free from exposure to said solid counter-ion source materialduring at least 1% of the charge-discharge cycles throughout thelifetime of said battery. (For example, a threshold concentration 10%less than the solubility limit of the sulfide-source material in anelectrolyte corresponds to a threshold concentrations that is 90% of thesolubility limit of the sulfide-source material in the electrolyte.) Insome embodiments, the threshold concentration is at least 1%, at least5%, at least 10%, at least 20%, at least 30%, at least 40%, at least50%, at least 60%, at least 70%, at least 80%, at least 90%, orpreferably for some applications substantially 95% less than thesolubility limit of the sulfide-source material in an identicalelectrolyte free from exposure to said solid counter-ion source materialduring at least 1%, at least 2%, at least 5%, at least 10%, at least20%, at least 50%, at least 75%, at least 80%, at least 90%, at least95%, or preferably for some applications at least 99% of thecharge-discharge cycles throughout the lifetime of said battery.

In some embodiments, the threshold concentration is substantially 1 µMduring at least 1% of the charge-discharge cycles throughout thelifetime of said battery. In some embodiments, the thresholdconcentration is substantially equal to 1×10⁻³ M, substantially equal to1×10⁻⁴ M, substantially equal to 1×10⁻⁵ M, substantially equal to 1×10⁻⁶M, or substantially equal to 1×10⁻⁷ M at 20° C. during at least 1%, atleast 2%, at least 5%, at least 10%, at least 20%, at least 50%, atleast 75%, at least 80%, at least 90%, at least 95%, or preferably forsome applications at least 99% of the charge-discharge cycles throughoutthe lifetime of said battery. In some embodiments, the dissolved sulfideion concentration in the electrolyte is substantially less than orsubstantially equal to 1 µM during at least 1% of the charge-dischargecycles throughout the lifetime of said battery. In some embodiments, thedissolved sulfide ion concentration in the electrolyte is substantiallyless than or substantially equal to 1×10⁻³ M, substantially less than orsubstantially equal to 1×10⁻⁴ M, substantially less than orsubstantially equal to 1×10⁻⁵ M, substantially less than orsubstantially equal to 1×10⁻⁶ M, or substantially less than orsubstantially equal to 1×10⁻⁷ M at 20° C. during at least 1%, at least2%, at least 5%, at least 10%, at least 20%, at least 50%, at least 75%,at least 80%, at least 90%, at least 95%, or preferably for someapplications at least 99% of the charge-discharge cycles throughout thelifetime of said battery. In some embodiments, the solid sulfide-sourcematerial is a first sulfide-source material characterized by itsdissociation into at least dissolved sulfide ions and dissolved firstcounter-ions upon its dissolution in said electrolyte; said batteryfurther comprising a second sulfide-source material operationallyconnected to the electrolyte; said second sulfide-source material beingsoluble in said electrolyte and characterized by its dissociation intoat least dissolved sulfide ions and dissolved second counter-ions uponits dissolution in said electrolyte; and wherein a solubility productconstant (“K_(sp)”) of said first solid sulfide-source material isgreater than a Ksp of said second solid sulfide-source material.

In some embodiments, a concentration of dissolved sulfide ions in theelectrolyte is greater than a solubility limit of CuS, Bi₂S₃, or CdS inan identical electrolyte otherwise free of dissolved sulfide ions andless than a solubility limit of ZnS, FeS, or MnS in an identicalelectrolyte otherwise free of dissolved sulfide ions. In someembodiments, the solid sulfide-source material is sparingly soluble inthe electrolyte. In some embodiments, the ratio of total moles ofdissolved and undissolved counter-ions in the battery to total moles ofdissolved and undissolved sulfur species in the battery is at least 1%,at least 5%, at least 10%, at least 20%, at least 30%, at least 40%, atleast 50%, at least 60%, at least 70%, at least 100%, at least 150%, atleast 200%, or preferably for some applications at least 210% than thestoichiometric ratio of counter-ions to sulfide ions in said solidsulfide-source material during at least 1%, at least 2%, at least 5%, atleast 10%, at least 20%, at least 50%, at least 75%, at least 80%, atleast 90%, at least 95%, or preferably for some applications at least99% of the charge-discharge cycles throughout the lifetime of saidbattery. In some embodiments, the sulfide ion concentration issubstantially less than or substantially equal to 0.1%, substantiallyless than or substantially equal to 0.2%, substantially less than orsubstantially equal to 0.5%, substantially less than or substantiallyequal to 1%, substantially less than or substantially equal to 5%, orsubstantially less than or substantially equal to 10% of the solubilitylimit of the sulfide-source material in an identical electrolyte free ofexcess dissolved counter-ions. In some embodiments, a concentration ofdissolved sulfide ions in the electrolyte is selected from the range of0.1 µM to 700 µM, selected from the range of 0.1 µM to 300 µM, greaterthan 0 and less than 1 µM, or selected from the range of 0.1 µM to 1 µM.In some embodiments, a concentration of dissolved sulfide ions in theelectrolyte is selected from the range of 0.1 µM to 700 µM, selectedfrom the range of 0.1 µM to 300 µM, greater than 0 and less than 1 µM,or selected from the range of 0.1 µM to 1 µM during at least 1%, atleast 2%, at least 5%, at least 10%, at least 20%, at least 50%, atleast 75%, at least 80%, at least 90%, at least 95%, or preferably forsome applications at least 99% of the charge-discharge cycles throughoutthe lifetime of said battery. In some embodiments, the sulfide compoundis incorporated sulfide, additive sulfide, or reservoir sulfide. In someembodiments, the sulfide-source material is incorporated sulfide,additive sulfide, or reservoir sulfide. In some embodiments, the sulfidecompound is a solid sulfide-source material.

In some embodiments, the solid sulfide-source material is selected fromthe group consisting of one or more metal sulfides, one or moresub-oxides of one or more metal sulfides, a solid solution of one ormore metal sulfides and one or more oxides or hydroxides, one or moresulfosalt minerals, one or more metalloid sulfides, one or more nonmetalsulfides, and any combination thereof. In some embodiments, the one ormore metal sulfides are selected from the group consisting of bismuthsulfide, iron sulfide, iron disulfide, iron-copper sulfide, zincsulfide, manganese sulfide, tin sulfide, copper sulfide, cadmiumsulfide, silver sulfide, titanium disulfide, lead sulfide, molybdenumsulfide, nickel sulfide, antimony sulfide, any polymorph of these, andany combination thereof. In some embodiments, the counter-ion sourcematerial is selected from the group consisting of one or more oxides,one or more hydroxides, and any combination thereof. In someembodiments, chemical equation of the solid sulfide-source materialcomprises Zn and S, and wherein chemical equation of the solidcounter-ion source material comprises Zn.

In some embodiments, the counter-ion source material is encapsulated inan encapsulation material, for example, an encapsulation material thatdissolves in the electrolyte at a slower rate than the dissolution rateof the counter-ion source material in the electrolyte. In someembodiments, the solid sulfide-source material is contained within asulfide reservoir. In some embodiments, the solid-sulfide sourcematerial is at least partially encapsulated by or bound within a bindermaterial. In some embodiments, the solid-sulfide source material issubstantially encapsulated by or bound within a binder material. In someembodiments, the solid-sulfide source material is at least partiallysandwiched between at least two electrically insulating layers, such aselectrically non-conductive polymer(s). In some embodiments, thesolid-sulfide source material is substantially sandwiched between atleast two electrically insulating layers. In some embodiments, thesulfide reservoir is in the form of a sheet comprising a non-conductivebinder or encapsulant at least partially encapsulating the solid-sulfidesource material. In some embodiments, the battery is an alkalinerechargeable battery and wherein the electrolyte is an aqueous alkalineelectrolyte.

The term “substantially” refers to a property or condition that iswithin 20%, within 10%, within 5%, within 1%, or is equivalent to areference property or condition. The term “substantially equal”,“substantially equivalent”, or “substantially unchanged”, when used inconjunction with a reference value describing a property or condition,refers to a value or condition that is within 20%, within 10%,optionally within 5%, optionally within 1%, optionally within 0.1%, oroptionally is equivalent to the provided reference value or condition.For example, a concentration is substantially equal to 1×10⁻⁶ M if thevalue of the concentration is within 20%, within 10%, optionally within5%, optionally within 1%, or optionally equal to 1×10⁻⁶ M. For example,a sulfide-source material is substantially encapsulated (an exemplarycondition) if at least 80%, at least 90%, at least 95%, at least 99%, oroptionally 100% of the sulfide-source material is encapsulated. The term“substantially greater”, when used in conjunction with a reference valueor condition describing a property or condition, refers to a value thatis at least 2%, optionally at least 5%, at least 10%, or optionally atleast 20% greater than the provided reference value or condition. Theterm “substantially less”, when used in conjunction with a referencevalue or condition describing a property or condition, refers to a valueor condition that is at least 2%, optionally at least 5%, optionally atleast 10%, or optionally at least 20% less than the provided referencevalue. For example, a concentration is substantially less than 1×10⁻⁶ Mif the value of the concentration is at least 20% less than, at least10% less than, at least 5% less than, or at least 1% less than 1×10⁻⁶ M.

Also disclosed herein are batteries comprising any one or anycombination of embodiments of the batteries disclosed herein. Alsodisclosed herein are methods of making batteries and methods ofoperating batteries comprising any one or any combination of embodimentsof the batteries disclosed herein.

Without wishing to be bound by any particular theory, there may bediscussion herein of beliefs or understandings of underlying principlesrelating to the devices and methods disclosed herein. It is recognizedthat regardless of the ultimate correctness of any mechanisticexplanation or hypothesis, an embodiment of the invention cannonetheless be operative and useful.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description sets forth illustrative embodimentswith reference to the accompanying drawings, of which:

FIG. 1 is a schematic illustration of a flooded sealed battery.

FIG. 2 is a schematic illustration of a sealed battery with apartially-flooded negative electrode extending into a headspace above anelectrolyte level and a flooded positive electrode submerged in theelectrolyte.

FIG. 3A is a schematic chart illustrating a relationship between sulfidecontent and discharge rate capability of an iron electrode.

FIG. 3B is a schematic chart illustrating loss of discharge capacity ofa nickel-iron battery with a small quantity of sulfide in the ironelectrode.

FIG. 3C is a schematic chart illustrating increasing rates of sulfideloss from an iron negative electrode at more negative potentials.

FIG. 4 is a schematic illustration showing a cross-section of amulti-layer structure that may be used as a recombination device or anauxiliary electrode.

FIG. 5 is a schematic illustration of a battery showing variousalternate positions of a recombination device.

FIG. 6 is a schematic illustration of a battery including an auxiliaryelectrode connected to a positive battery electrode.

FIG. 7 is a schematic illustration of a battery including an auxiliaryelectrode connected to a negative battery electrode.

FIG. 8 is a schematic illustration of a battery including a firstauxiliary electrode connected to a positive battery electrode and asecond auxiliary electrode connected to a negative battery electrode.

FIG. 9 is a schematic illustration of a transverse cross-section of adual auxiliary electrode structure.

FIG. 10 is a schematic illustration of a longitudinal cross-section of adual auxiliary electrode structure.

FIG. 11 is a schematic illustration of a battery including an auxiliaryelectrode connected to both a positive battery electrode and a negativebattery electrode.

FIG. 12 is a schematic illustration of a battery including arecombination device configured for catalytic combustion of hydrogen andoxygen.

DETAILED DESCRIPTION Introduction and Definitions

The various embodiments will be described in detail with reference tothe accompanying drawings. References made to particular embodiments,examples, or implementations are for illustrative purposes, and are notintended to limit the scope of the claims. The following disclosurerelates to batteries and battery components.

As used herein, the term “battery” refers to an energy-storingelectrochemical device comprising at least one electrochemical cell in abattery container. For example, a battery may include only oneelectrochemical cell (i.e., at least one positive electrode and at leastone negative electrode joined by an ion-transfer medium) in a batterycontainer. In other examples, a battery may include a bipolar stack orother arrangement of two or more electrochemical cells in one batterycontainer. The term “battery pack” will generally be used herein torefer to units made up of two or more batteries electrically joined toone another to perform as a collective unit.

The term “electrochemical cell” used herein refers to devices and/ordevice components that convert chemical energy into electrical energyand/or electrical energy into chemical energy. Electrochemical cellshave two or more electrodes (e.g., positive and negative electrodes) andan electrolyte, wherein electrode reactions occurring at the electrodesurfaces result in charge transfer processes. Electrochemical cellsinclude, but are not limited to, primary batteries, and secondarybatteries. General cell and/or battery construction is known in the art,see e.g., U.S. Pat. Nos. 6,489,055, 4,052,539, 6,306,540, Seel and DahnJ. Electrochem. Soc. 147(3) 892-898 (2000).

The term “electrode” used herein refers to an electrical conductor whereions and electrons are exchanged with electrolyte and an outer circuit.The terms “positive electrode” and “cathode” are used synonymously inthe present description and refer to the electrode having the higherelectrode potential in an electrochemical cell (i.e. a more positivepotential than the negative electrode). The terms “negative electrode”and “anode” are used synonymously in the present description and referto the electrode having the lower electrode potential in anelectrochemical cell (i.e. a potential less positive than the positiveelectrode). The terms “cathodic reduction” or “electrochemicalreduction” refer to a gain of electron(s) of a chemical species, and theterms “anodic oxidation” or “electrochemical oxidation” refer to theloss of electron(s) of a chemical species.

The term “electrolyte” refers to an ionic conductor which can be in thesolid state, the liquid state (most common) or more rarely a gas (e.g.,plasma). Liquid electrolytes may include aqueous solutions, non-aqueoussolutions, ionic liquids, molten salts, or others.

Most batteries with aqueous liquid electrolytes generate gas as a resultof side-reactions which may occur during charging, discharging, orduring open-circuit stand. The particular gas-generating side reactionsgenerally depend on the battery’s chemistry and operating conditionssuch as charge rates and/or discharge rates, temperature, and otherfactors.

Example gas generation reactions (also referred to as “gas evolution” or“electrolysis” reactions) are described below, but other gas-generationand/or hydrolysis reactions may occur instead of or in addition to thesein various embodiments.

Hydrogen gas (H₂) may be generated on a negative electrode according to:

Hydrogen Evolution Reaction: 2H₂O + 2e⁻ → H₂ + 2OH⁻

Hydrogen gas may also be generated by self-discharge of an iron negativeelectrode according to:

Iron Self-Discharge Reaction: Fe + 2H₂O → Fe(OH)₂ + H₂

Oxygen gas (O₂) may be evolved during charging according to:

Oxygen Evolution Reaction: 4OH⁻ → O₂ + 2H₂O+ 4e⁻

For example, lead-acid batteries tend to electrolyze water from theaqueous acidic electrolyte, particularly during over-charging.Over-charging is defined as continuing to apply a charging current to abattery after one or both battery electrodes have reached afully-charged capacity (i.e., all available chemical species having beenconverted to an energy-storing form). Similarly, nickel cadmiumbatteries tend to electrolyze water from the alkaline aqueouselectrolyte. In general, batteries with aqueous electrolytes tend togenerate oxygen at the positive electrode during charging and may alsoproduce hydrogen at the negative electrode. The quantity of each gasproduced will tend to vary for different electrode materials,electrolyte compositions, and other properties and operatingcharacteristics of a battery and its components. For example, hydrogengeneration from a cadmium negative electrode can generally be avoided bypreventing the negative cadmium electrode from being over-charged.

In some cases, the produced gases are simply vented out of the batteryto the surrounding atmosphere. However, continued electrolysis ofelectrolyte water into gas causes the electrolyte level to drop andchanges the electrolyte composition, requiring regular topping-up ofwater. Such maintenance may be costly and burdensome, and failure toperform adequate water additions at the correct times may cause thebattery to quickly degrade and fail.

Additionally, any efficiency mis-match between the positive and negativeelectrodes may require taking the battery out of service for regularrebalancing operations or other maintenance operations. These problemsmay generally be avoided by sealing a battery to prevent the release ofproduced gases.

However, to utilize a gas-producing battery in a sealed configuration,the production of gas is preferably minimized or avoided and/or theproduced hydrogen and/or oxygen gases is preferably recombined intowater or other species that may ultimately return the hydrogen andoxygen atoms to a form usable by the battery. Without recombination ofgases, internal pressure within a sealed battery container will continueto rise, eventually causing the container to release the gases, andpossibly to explode.

As used herein, the term “recombine” refers to chemical orelectrochemical reactions in which hydrogen gas, oxygen gas, or both areconverted into water, a watersoluble form (e.g., a hydroxide ion) or asolid compound on a battery electrode such as in the form of a metaloxide or a metal hydroxide.

Example gas recombination reactions are described below, but other gasrecombination reactions may occur instead of, or in addition to these invarious embodiments.

Stoichiometric (2:1) mixtures of hydrogen and oxygen gases may bere-combined by combustion to form water. In some cases, catalysts may beused to lower the temperature at which hydrogen-oxygen combustion occursspontaneously.

Hydrogen-Oxygen Combustion Reaction: 2H₂ + O₂ → 2H₂O

Oxygen may be electrochemically reduced with water to form solublehydroxide (OH⁻) ions according to:

Oxygen Reduction Reaction (ORR): O₂ + 2H₂O + 4e⁻ → 4OH⁻

Hydrogen gas may be oxidized to soluble hydrogen (H⁺) ions according to:

Hydrogen Oxidation Reaction (HOR): H₂ → 2H⁺ + 2e⁻

So-called “recombinant” lead-acid batteries are configured to performgas recombination internally so that they may be sealed with a valve torelease gas when pressure exceeds a pre-set value. In order to simplifythe gas management problem, recombinant lead-acid batteries tend to havenegative electrodes with larger capacities than positive electrodes soas to substantially prevent the negative electrodes from beingover-charged, thereby substantially preventing generation of hydrogen.This leaves predominantly only produced oxygen to be managed.

A similar approach is taken in nickel-cadmium batteries designed tooperate primarily or solely on an oxygen cycle. The main solution in thecase of nickel-cadmium batteries has been to use a “starved”configuration in which a battery contains only a very small quantity ofelectrolyte, typically relying on capillary action to absorb electrolyteinto pores of the electrodes and/or separator. By reducing the quantityof electrolyte in the battery, substantial portions of the negativeelectrode are left exposed to the produced oxygen. This configurationencourages recombination of oxygen on exposed portions the negativeelectrode. In the case of both recombinant lead-acid and recombinantnickel cadmium batteries, the quantity of hydrogen produced is expectedto be small by design, thereby eliminating or substantially reducing theneed to recombine hydrogen gas.

Therefore, existing “sealed” or “recombinant” batteries are generallyengineered to simplify the gas recombination problem to a narrow set ofconditions. That is, many batteries are designed such that the need forhydrogen recombination can be nearly or entirely ignored, and oxygenrecombination can reliably be expected to occur on the negative batteryelectrode. Alternatively, some batteries are designed to operate on thehydrogen cycle, minimizing any need for oxygen recombination whileperforming hydrogen recombination internally.

On the other hand, these simplifications are in some instances notcompatible with a fully or partially-flooded alkaline battery systemsuch as a nickel-iron battery, a nickel-zinc battery, or a manganesedioxide-iron battery (among others). In such cases, oxygen may stilltend to be generated at the positive electrode, particularly duringover-charging of the positive electrode. In the same system, hydrogenmay be generated at the negative electrode during all stages ofcharging, but especially during over-charging and high-rate charging.

In some batteries, such as those with iron or zinc negative electrodes,hydrogen may also be generated through self-discharging reactions. Infact, iron negative electrodes may generally produce hydrogen at anystate-of-charge, while charging, discharging, or at open-circuit as longas metallic iron is present. In a nickel-iron battery, significantquantities of both hydrogen and oxygen will tend to be produced duringeach charging cycle, thereby uniquely complicating the objective ofproducing a sealed recombinant battery.

The constraints placed on recombinant lead-acid and recombinant nickelcadmium batteries also substantially limit their longevity as well astheir robustness to operation in extreme conditions. Starved-electrolytebatteries are generally incapable of cycle lives longer than severalhundred cycles because the limited quantity of electrolyte eventuallydries out, leaving insufficient electrolyte to support electrochemicalreactions. Limited quantities of electrolyte may also reduce thecharging and discharging rate capabilities of starved batteries.

Flooded batteries and semi-flooded batteries, which contain a largervolume of electrolyte relative to the electrodes, are generally capableof much longer lives and are less susceptible to thermal runaway. Asused herein, the term “flooded” or “flooded electrolyte” may broadlyrefer to any battery system in which a volume of electrolyte issufficient to fill a battery container to a level at least as high asthe top-most portion of at least one battery electrode such that theelectrode is entirely submerged in the electrolyte. As used herein, theexpression “entirely submerged in the electrolyte” refers to aconfiguration wherein all external surfaces of a structure or devicecomponent, such as an electrode, are surrounded by the electrolyte,including configurations wherein all portions of the structure or devicecomponent, such as an electrode, are provided below the electrolytelevel.

In various situations, a battery may be “flooded” or “semi-flooded” withrespect to a specific electrode or with respect to all electrodes withinthe battery. For example, a battery may be “flooded” with respect to aparticular electrode if that electrode is entirely submerged below theelectrolyte level within the battery container. A battery may be“semi-flooded” or “partially flooded” with respect to a particularelectrode if some portion of that electrode extends above theelectrolyte level in the battery container such that only a portion ofthe electrode is submerged in the electrolyte. Flooded and semi-floodedbatteries may be distinguished from so-called “starved” cells whichcontain a very small quantity of liquid or gel electrolyte present onlywithin pores of the electrodes and/or absorbed into a separator.Nonetheless, any of the embodiments described herein may be applied tobatteries or cells in flooded, semi-flooded, or starved electrolyteconfigurations.

Some batteries may be configured to operate in a particular orientationrelative to gravity such that the battery’s “top” is the region furthestfrom the center of the gravitational field holding the electrolyte“down.” The terms and definitions used herein generally assume that theorientation of the battery used to define a “flooded” or “semi-flooded”electrolyte quantity is the same as an orientation in which the batteryis operated.

If a flooded or semi-flooded battery can be sealed, then severalimprovements may be achieved compared to starved or vented batteries. Asealed flooded battery may be capable of a substantially longer life,increased safety resulting from decreased release of flammable gas, anddecreased maintenance in the form of water or electrolyte additions.These benefits may allow for long-term use of such batteries inapplications or locations in which regular replacement or maintenance ofbatteries is impractical, costly, or impossible.

In some battery systems it may be possible to simplify the gasrecombination problem by configuring and/or operating a battery toincrease predictability of the composition and/or rate of gasproduction. For example, in some systems it may be possible to controlthe battery’s operation and/or construction such that very littlehydrogen is produced, thereby requiring predominantly only oxygenrecombination. In other systems, it may be possible to control thebattery’s operation and/or construction such that only stoichiometricquantities of hydrogen and oxygen are produced, thereby requiring only acombustion reaction to form water.

However, in some battery systems, up to three different gas conditionsmay occur, each of which may require a different recombination approach.These gas conditions include a “stoichiometric” condition, anoxygen-rich condition, and a hydrogen-rich condition. In astoichiometric condition, the quantity of hydrogen gas molecules isapproximately twice the quantity of oxygen gas molecules, i.e., in aratio roughly equal to the stoichiometric relationship of atoms inwater: H₂O. In an oxygen-rich condition, the quantity of oxygen gassubstantially exceeds the quantity of hydrogen gas (if any hydrogen gasis present at all). Similarly, a gas condition may be hydrogen-rich ifthe quantity of hydrogen gas substantially exceeds the quantity ofoxygen, if any.

In order to provide a robust, long-lived battery system, a flooded orsemi-flooded sealed battery may be capable of handling any of thesethree gas conditions with a high degree of reliability and at a minimumof cost and complexity. In some cases, a combination of multiple gasmanagement approaches may be desirable in order to produce a robustbattery system. Therefore, in various embodiments, any one or more ofthe technologies described herein for gas evolution mitigation, directrecombination, auxiliary electrode recombination, oxygen/hydrogencombustion, or external fuel cell recombination may be combined with oneor more others to form a robust sealed battery system. All suchcombinations are intended to fall within the scope of this disclosure.

For some applications, approaches to gas management may be organizedinto six groups: (1) minimizing gas evolution; (2) direct recombinationof a gas on a battery electrode; (3) auxiliary oxygen reductionelectrodes; (4) auxiliary hydrogen oxidation electrodes; (5)hydrogen/oxygen combustion elements; and (6) external fuel cells.

More broadly, gas recombination may be performed either directly withactive material of one or more battery electrodes, or by use of an addedrecombination device. As used herein, the term “recombination device”may broadly include any device component, structure or material thatsupports one or both of chemical and electrochemical gas recombinationreactions and that exists in addition to active materials of batteryelectrodes. For some applications, therefore, a recombination device maybe an auxiliary electrode supporting electrochemical gas oxidationand/or reduction reactions, a hydrogen/oxygen combustion element, anelement supporting or catalyzing chemical oxidation and/or reductionreactions, a fuel cell, or a mass of material carried by a batteryelectrode for the purpose of supporting chemical or electrochemical gasrecombination reactions.

The generation of gas can be minimized by controlling operatingconditions of a battery, such as temperature, charging or dischargingrates, depth of charging or discharging, or others. Gas generation canalso be mitigated by including additives in an electrode or anelectrolyte so as to make gas-generating side-reactions less likely tooccur. Various examples of gas minimizing structures and operations aredescribed herein.

Direct recombination involves chemical or electrochemical reactionsbetween a generated gas and battery electrode active material (i.e., notan auxiliary electrode as defined herein) resulting in the gas beingconverted to water, a soluble species, or other compound. Directrecombination may be encouraged by structural or compositional featuresof electrodes, separators, electrolytes, battery containers, or otheraspects of a battery. Various examples of structures and compositionsthat encourage direct gas recombination on one or more batteryelectrodes are described herein.

As used herein, the term “auxiliary electrode” generally includes anyelectrochemically active material that is distinct from, or in additionto, the active material of the battery electrodes performing primaryenergy storage operations (e.g., the positive and negative electrodes).In some embodiments, for example, an auxiliary electrode may beelectrically connected (or connect-able via a switch, diode, or otherelectronic component) to a battery electrode or other structure orelement imparting an electric potential to the auxiliary electrode. Insome cases, an auxiliary electrode may also support or catalyze chemicaloxidation, reduction, or combustion reactions.

For example, some auxiliary electrodes may comprise an auxiliary activematerial carried by a substrate that is physically separated (e.g., notin physical contact with) and independent of the battery positive andnegative electrode current collectors and active materials. In otherexamples, an auxiliary electrode may comprise a battery electrodecurrent collector or other battery electrode structure coated with anauxiliary active material. In further examples, an auxiliary electrodemay comprise an auxiliary active material added to a section of abattery electrode active material or other material or structure withinthe battery container. In one example, an auxiliary electrode maycomprise an auxiliary active material supported by or applied to aportion of a battery container interior wall.

As the term is used herein, “active material” of a battery electrodeincludes a material having a primary function in a battery to undergooxidation or reduction in order to store or deliver energy. Similarly,“auxiliary active material” may include a material or catalyst whoseprimary function is to catalyze or otherwise support electrochemical gasrecombination reactions.

As used herein, the term “auxiliary oxygen reduction electrode” refersto an auxiliary electrode that supports or catalyzes electrochemicalreactions involving the reduction of oxygen, such as reduction ofmolecular oxygen gas (O₂). Similarly, the term “auxiliary hydrogenoxidation electrode” refers to an auxiliary electrode that supports orcatalyzes electrochemical reactions involving the oxidation of hydrogen,such as oxidation of molecular hydrogen gas (H₂). In some embodiments, asingle auxiliary electrode structure may support both reactions, whilein other embodiments, separate auxiliary electrode structures may beused to provide for oxidation and reduction reactions separately.Various examples of auxiliary electrode structures, compositions, andmethods of operation are described herein.

Hydrogen/oxygen combustion elements may generally include any materialor structure that supports or catalyzes the combustion of hydrogen gaswith oxygen to produce water. In some embodiments, an auxiliaryelectrode or a portion of a battery electrode may also act as ahydrogen/oxygen combustion element. Various examples of hydrogen/oxygencombustion structures are described herein, although the examplesdescribed herein are not necessarily exhaustive.

In some embodiments, gasses may be vented or otherwise extracted from abattery container to a separate space in which the gases may bechemically or electrochemically recombined. In some embodiments, suchseparate spaces may be used to combine gasses produced from multiplebatteries or from multiple battery packs. In some embodiments, chemicalor electrochemical gas recombination may be performed using one or morefuel cell devices within the battery container or external to thebattery container to oxidize hydrogen and/or reduce oxygen.

Also disclosed herein are batteries having any one or any combination ofembodiments of batteries disclosed herein. Also disclosed herein aremethods having any one or any combination of embodiments of methods andbatteries disclosed herein. For example, also disclosed herein aremethods for making batteries according to any one or any combination ofembodiments disclosed herein. Also disclosed herein are methods forcharging or discharging batteries according to any one or anycombination of embodiments disclosed herein.

Battery Structures and Direct Recombination

FIG. 1 illustrates a secondary (rechargeable) battery system 10comprising a positive electrode 12, a negative electrode 14, and aseparator 16 within a battery container 18 filled with electrolyte 20 toa level 22 at least as high as the tops 32, 34 of the electrodes 12, 14.The space above the electrolyte level 22 may be referred to as theheadspace 24. The positive electrode 12 may be electrically connected tothe battery’s positive terminal 42 and may contain active material thatmay undergo reduction reactions during discharging and oxidationreactions during charging. The negative electrode 14 may be electricallyconnected to the battery’s negative terminal 44 and may contain activematerial that may undergo oxidation reactions during discharging andreduction reactions during charging of the battery 10.

In various embodiments, the positive electrode 12 active material mayinclude metals or metal oxides such as one or more of nickel oxide,nickel hydroxide, manganese dioxide, silver, or other metals or metaloxides. In some embodiments, the positive electrode may also include oneor more additives for enhancing conductivity, performance, limiting gasproduction, or other objectives. Examples of such additives may includecarbon, graphite, zinc, manganese, manganese dioxide, yttrium,cobalt-containing additives such as cobalt hydroxide, cobaltoxyhydroxide or cobalt oxides, or others. Additives may include oxides,hydroxides, salts, or other compounds of these or other metals.Additives may include non-metals such as carbon black, graphite, orinert materials (e.g., polymers or ceramics) coated with metals, metaloxides, or metal hydroxides. In various embodiments, any combination ofthese or other materials may be used as positive electrode additives.

In some embodiments, the positive electrode and/or the negativeelectrode may comprise a binder material for binding active materialparticles and additive particles to one another and/or to a substrate(e.g., a current collector). Various binder materials may be used ineither or both electrodes. For example, various polymers, alcohols(e.g., polyvinyl alcohol), rubbers (e.g., styrene butadiene rubber orothers), ethyl cellulose, methyl cellulose, latex, epoxide resins. Insome embodiments, a hydrophobic polymer binder may beneficially assistin transporting gas produced by an electrode out of the electrode to theheadspace.

For example, a battery with hydrophobic material both a positiveelectrode and an iron-containing negative electrode may aid thetransport of oxygen gas produced on the positive electrode across theseparator and into the negative electrode where the oxygen may berecombined by reaction with the iron active material.

In various embodiments, other hydrophobic materials (e.g., in additionto or instead of PTFE) may be used as a binder or as a hydrophobicadditive in positive and/or negative electrodes. Examples of otherhydrophobic materials may include fluorinated ethylene propylene (FEP),or perfluoroalkoxy alkanes (PFA), acrylics, amides, imides, carbonates,dienes, esters, ethers, fluorocarbons, olefins, styrenes, vinyl acetals,vinyl and vinylidene chlorides, vinyl esters, ketones, vinylpyridinepolymers, and vinypyrrolidone polymers, among others. Furtherhydrophobic and superhydrophobic materials and treatment processes aredescribed in “Superhydrophobic Polymers” by Melanie Wolfs, ThierryDarmanin, and Frederic Guittard, published online in the Encyclopedia ofPolymer Science and Technology on 12 Jul. 2013. Such materials and/ortreatments, or combinations thereof, may also be used in electrodesdescribed herein.

In some embodiments, a hydrophobic polymer such as PTFE may befibrillated by application of a shear force prior to or duringfabrication of an electrode. Fibrillating shear forces may be applied byspinning PTFE powder particles in a speedmixer, by passing PTFEparticles (optionally mixed with other electrode active materials and/oradditives) through compression rollers or otherwise compressing the PTFEparticles.

In some embodiments, a negative or positive electrode may contain aconductive polymer additive such as polyaniline, polycyclic aromaticcompounds, polypyrrole, a polyacetylene, polyindole, or others.

The negative electrode 14 active material may include metal or metaloxides such as iron, zinc, cadmium, or other metals and/or oxides orhydroxides of these or other metals. In some embodiments, the ironnegative electrode active material may include iron provided aselemental iron and/or as an iron-containing material, such as aniron-containing alloy or an iron-containing compound, such as an ironoxide, iron mixed oxide, iron hydroxide, iron sulphate, iron carbonate,iron sulfide, or any combination of these. In some embodiments, ironnegative electrode active materials may include purified or refined ironmaterials such as carbonyl iron or electrolytic iron, or iron ores suchas magnetite, maghemite, iron carbonate, hematite, goethite, limonite,or other iron materials.

In some embodiments, the iron negative electrode active material furthercomprises one or more negative electrode additives. In some embodiments,negative electrode additives may include a sulfide compound (e.g.,bismuth sulfide, iron sulfide, iron disulfide, iron-copper sulfide, zincsulfide, manganese sulfide, tin sulfide, copper sulfide, cadmiumsulfide, silver sulfide, titanium disulfide, lead sulfide, molybdenumsulfide, nickel sulfide, antimony sulfide, dimethylsulfide, carbondislulfide, etc.) and/or a bismuth-containing compound (e.g., bismuthsulfide or bismuth oxide). In some embodiments, one or more sub-oxidesof iron sulfide of the form FeS₁-_(x)O_(x) and/or one or more solidsolutions of iron sulfide and iron hydroxide of the form FeS_(X)(OH)_(Y)may be included as an iron negative electrode additive.

In some embodiments, an iron negative electrode may contain carbonyliron or other iron active material (e.g., magnetite, hematite, or otheriron oxides or iron hydroxides) and two or more soluble metal sulfideadditives in amounts from about 0.01 weight percent (as a percent of theweight of carbonyl iron) to 10 weight percent or more. For example, aniron negative electrode may contain an iron active material, an ironsulfide additive in an amount from about 0.01% to about 10% by weight ofthe iron active material, and a second sulfide compound (e.g., bismuthsulfide, iron sulfide, iron disulfide, iron-copper sulfide, zincsulfide, manganese sulfide, tin sulfide, copper sulfide, cadmiumsulfide, a sub-oxide of iron sulfide, silver sulfide, titaniumdisulfide, lead sulfide, molybdenum sulfide, nickel sulfide, antimonysulfide, dimethylsulfide, carbon dislulfide, or others) in an amountfrom about 0.01% to about 10% by weight of the iron active material.

In some embodiments, a negative electrode may include one or moreadditive materials selected for enhancing conductivity, inhibiting gasevolution, improving discharge rate capability, or other batteryperformance enhancing objectives. Some examples of such additives mayinclude metals such as copper, cobalt, nickel, tin, antimony, bismuth,indium, silver, gold, lead, or cadmium. Additives may include oxides,hydroxides, sulfides, salts, or other compounds of these or othermetals, or elemental materials such as elemental sulfur. Additives mayinclude non-metals such as carbon black, graphite, or inert materials(e.g., polymers or ceramics) which may be coated with metals, metaloxides, or metal hydroxides. In various embodiments, any combination oftwo or more these or other materials may be used as negative electrodeadditives. Suitable negative electrode materials and fabricationprocesses are described in U.S. Pat. 9,577,298 and U.S. Pat. ApplicationPublication 2015/086884, both of which are incorporated herein byreference.

While examples and embodiments are described herein with reference tonickel positive active materials and iron negative active materials, thevarious embodiments may also be used in combination with other activematerials or combinations of active materials.

In various embodiments, the electrodes 12, 14 may be made by anysuitable process, such as sintering, hot-pressing, cold-pressing,wet-paste lamination, dry pressing, slurry coating, PTFE based process,roll bonding, tape casting (blade coating), or combinations of these orany other suitable process. In various embodiments, the electrodes mayalso include additive materials, pore formers, binders, currentcollectors, conductivity-enhancing additives, or other materials.

Pore formers may include particulate materials that may be included inan electrode during construction and that may be removed by a later stepsuch as soaking in water, electrolyte, or other solvent and/or bysintering the constructed electrode (also referred to as an “as-made”electrode). Pore formers may include materials soluble in water orelectrolyte such as polyvinyl alcohols, sodium bicarbonate, camphor,starches, or electrolyte-soluble metal oxides or hydroxides (e.g., tinoxide, zinc oxide, lead oxide, copper oxide, manganese oxide).

In some embodiments, a positive or negative electrode may be made bypressing active materials (hot, cold, wet, or dry) into or onto asubstrate or “current collector” material. Such current collectors mayinclude perforated metal sheets, solid metal sheets, sheets of wovenmetal fibers, metal mesh, metal foam, fibrous metal wool structures, orother conductive metal substrates. Metal substrates may be made ofsteel, nickel, iron, copper, titanium, silver, nickel ferrite, cobaltferrite, spinel-coated materials, or combinations of these or othermetals (e.g., steel or iron coated with nickel, copper, or othermetals). In some embodiments, a current collector substrate may comprisean inert material coated with a conductive material, such asmetal-coated polymers or metal-coated ceramics.

Positive and/or negative electrodes may undergo “formation” processing(or “formation cycling”) in order to cause micro-structural,morphological and/or electrochemical changes in the material of theelectrode relative to an “as-made” electrode. The term “as-madeelectrode” refers to a fabricated electrode prior to any formationprocessing or other electrochemical cycling. The term “formationprocessing” generally refers to a process in which an as-made electrodeis assembled in a formation cell with a counter-electrode. The formationcell is then subjected to electrochemical cycling by charging anddischarging the cell until a desired endpoint. In various embodiments,the endpoint of a formation process may be defined in terms of a changein capacity at the end of a discharging cycle. For example, formationmay be ended when measured discharge capacity of the forming electrodechanges by less than a pre-determined amount (e.g., 5%, 2%, 1%, 0.5%)from one discharge cycle to the next. An average capacity of some numberof final formation cycles is generally referred to as the“end-of-formation capacity” or simply “formation capacity” of theelectrode. For example, an electrode’s formation capacity may be theaverage discharge capacity of the final 2, 3, 4, 5 or more cycles offormation processing.

“Capacity” of a battery electrode may be defined in various ways(capacity of auxiliary gas recombination electrodes is describedseparately below). In general, an electrode’s capacity is the totalenergy that may be stored in and delivered from that electrode. Inpractical terms, an electrode’s theoretical capacity is typically largerthan its “rated” capacity, which may be different than its“end-of-formation” capacity, which may be different than a measuredcapacity in any given cycle. A battery generally cannot spontaneouslydeliver more energy than is stored in the smaller (lower-capacity)electrode, so the capacity of a battery will tend to be descriptive ofthe lower-capacity electrode within the battery, also referred to as the“limiting” electrode.

An electrode’s theoretical capacity is a measure of the electrode’scapacity assuming 100% of the total active material in the electrode iscapable of being charged and discharged. However, in practical terms,less than 100% of the active material will be usable due to variousfactors such as electrical connectivity of the particles, the formationprocess used, discharge rate limitations, and other factors. The term“chargeable capacity” is used herein to refer to the total amount ofenergy that may be stored in an electrode or battery regardless of howmuch may be delivered at any particular rate. Battery electrodes thatare manufactured under consistent conditions are typically given a“rated” capacity that is typically a minimum capacity that a customermight expect to see. The rated capacity is often less than theend-of-formation capacity.

The term “discharge capacity” may generally refer to the quantity ofenergy delivered by a battery (or electrode) after the battery (orelectrode) is charged to a particular “charge capacity” which may begreater or less than the theoretical, rated, or end-of-formationcapacity. The magnitude of the discharge capacity divided by themagnitude of the charge capacity is referred to as the “coulombicefficiency” of the battery or electrode. Discharge capacity may also beaffected by the rate of discharge. For example, some electrodes maydeliver more capacity at slower discharge rates (i.e., lower currentdensities) than the same electrodes may deliver at faster dischargerates (i.e., higher current densities). The discharge capacity at anygiven rate may be referred to as the “accessible capacity” of theelectrode at that rate. The term “discharge rate” refers to the currentat which an electrochemical cell or battery is discharged. Dischargerate can be expressed in units of ampere. Alternatively, discharge ratecan be normalized to the rated (or other) capacity of the battery, andexpressed as C/(X t), wherein C is the capacity of the electrochemicalcell, X is a variable and t is a specified unit of time, as used herein,equal to 1 hour.

As the term “capacity” is used herein, it may refer to any of these orother definitions of capacity unless a particular definition isspecified. When discussing a ratio of capacities, it is more importantto use the same capacity definition for both electrodes in the ratiothan which definition of capacity is used. Therefore, if a capacityratio is specified herein, we assume only that both sides of the ratiouse the same definition of capacity as one another.

In various embodiments, the electrolyte 20 may be an aqueous ornon-aqueous alkaline, neutral, or acidic solution. For example, theelectrolyte solution may contain potassium hydroxide (KOH), sodiumhydroxide (NaOH), lithium hydroxide (LiOH) or combinations of these. Insome embodiments, the electrolyte may contain further additives such assurfactants (e.g., Triton X-100), corrosion inhibitors, dissolvedsulfides (e.g., sodium sulfide or potassium sulfide), or others. In someexample embodiments, an electrolyte may contain 5 to 15 M KOH and 0.1 to2% (w/v) LiOH.

In some embodiments, a battery 10 may include a separator 16 configuredto allow transfer of ions between the electrodes 12, 14 via theelectrolyte. In some embodiments, a separator may be chosen based on anability to allow selective transfer of desired molecules or materialswhile substantially limiting or preventing transfer of undesiredmolecules or materials. For example, some separator membranes areion-selective and allow the transfer of negative (or positive) ionswhile substantially preventing transfer of positive (or negative) ions.In other examples, separator materials may be chosen based on an abilityto allow or prevent the cross-over of gas bubbles from one side(associated with one electrode) to the opposite side (associated withthe counter-electrode).

In various embodiments, a battery separator may have propertiesbeneficial for an efficient long-life battery. Such properties may beproperties of materials from which the separator is formed or propertiesengineered into the separators by various processing steps, additivematerials, or other modifications. Examples of properties beneficial foran iron-electrode battery separator may include stability in a chosenelectrolyte (e.g., an alkaline aqueous solution), hydrophilicitysufficient to allow efficient ionic transport and to substantiallyprevent gas bubbles from crossing through the separator, patterns ofhydrophobicity for directing gas bubbles toward a desired region of thecell, a diffusion coefficient low enough to slow transport of sulfideions but high enough to allow adequate transport of electrolyte ionssuch as hydroxide ions, phase-selectivity for disallowing transport ofgas bubbles, and/or other properties. In some embodiments, a separatorused in an iron-electrode battery may beneficially have a diffusioncoefficient (for sulfide or hydroxide) of at least about 1 ×10⁻⁸ cm²/s(e.g. NAFION ® or ZIRFON ® separators), but less than about 1 ×10⁻⁶cm²/s, (e.g., a compressed polyolefin membrane, a polyvinyl alcoholmembrane (with or without cross-linking agents), an ethyl-cellulose ormethyl-cellulose membrane, or others).

In various embodiments, a separator 16 may be made of one or more ofvarious materials, including nylon, polyethylene (PE), polypropylene(PP), polyolefins (PO), polyamide (PA), poly(tetrafluoroethylene)(PTFE), polyvinylidine fluoride (PVdF), poly(vinyl chloride) (PVC),polysulfone (PSU), polyphenylsulfone (PPSU), polyetheretherketone(PEEK), asbestos, zirconium oxide cloth, cotton, polyvinyl alcohol orpolyvinyl acetate (PVA), ethyl-cellulose, methyl-cellulose,ethylene-methacrylic acid copolymers, fluorinated polymers, sulfonatedpolymers, carboxylic polymers, woven or nonwoven cellulose, NAFION, orothers. In some embodiments, a separator material may be modified byaddition of cross-linking agents or by post-treatments such as coronadischarge treatments for modifying surface features of the material suchas modifying a hydrophilicity or hydrophobicity of the material. In someembodiments, ceramic membranes or composite ceramic/polymer membranesmay also be used.

In some embodiments, a separator may be made of a suitable material andconstruction so as to be substantially hydrophilic and impervious to oneor more gases. In some embodiments, separators may be made of materialsfrom the class of materials known as ionomers, including anion exchangemembranes and proton exchange membranes, which may be solid non-porousmaterials capable of conducting ions without allowing diffusion ordirect flow of liquids or dissolved species through them. Examples ofionomers include ethylene-methacrylic acid copolymers such as thatproduced by DUPONT under the trademarks SURLYN and NUCREL,fluoropolymer-copolymers such as that produced by DUPONT under thetrademark NAFION, or others.

In some embodiments, separators may include solid-gel materials orcomposite materials such as the separator materials described in U.S.Pat. Application Publications US20020012848, US20020010261,US20030099872, and US20120148899, U.S. Pats. US3953241, US6358651, andUS6183914, or European Patent EP0624283B1. For example, a compositematerial separator membrane may comprise a polymer membrane (e.g., madeof one or more of the materials described above) impregnated with ametal oxide or metal hydroxide (e.g, oxides, dioxides, sub-oxides, orhydroxides of metals such as zirconium, aluminum, lithium, titanium,magnesium, etc.).

In some embodiments, a separator material may be treated to increase ordecrease hydrophilicity or hydrophobicity as described in “Use of aCorona Discharge to Selectively Pattern a Hydrophilic/HydrophobicInterface for Integrating Segmented Flow with Microchip Electrophoresisand Electrochemical Detection,” by Laura A. Filla, Douglas C.Kirkpatrick, and R. Scott Martin in Analytical Chemistry, 2011, 83 (15),pp 5996-6003 on 1 Aug. 2011. Alternatively, US Patent Applications2012/0328905 and 2015/0136226 describe methods for forming hydrophilicsurfaces by laser treatments which may be adapted for creatinghydrophobic regions in separator materials or electrode materials asdescribed herein.

In various embodiments, separator materials may be selected or modifiedin order to allow or prevent oxygen or other gas to pass through theseparator while maintaining electrical separation of the electrodes. Forexample, various microporous separator materials may allow oxygen gas topass through. Separator materials not generally capable of allowing thetransmission of oxygen gas may be made permeable to oxygen gas byperforating the material or otherwise forming pores for the transmissionof gases. In some embodiments, a separator 16 may be omitted and/orreplaced by another ionically conductive pathway.

In various embodiments, the battery container 18 may be made of anysuitable materials and construction capable of containing theelectrolyte, electrodes, and at least a minimum amount of gas pressure.For example, the battery container 18 may be made of metals, plastics,composite materials, or others. In some embodiments, the batterycontainer 18 may be sealed so as to prevent the escape of any gasesgenerated during operation of the battery.

In some embodiments, the battery container 18 may include a pressurerelief valve to allow release of gases when a gas pressure within thebattery container 18 exceeds a pre-determined threshold. In someembodiments, the threshold gas pressure for prismatic batteries may beany pressure up to about 50 PSIG or more. In some embodiments, thethreshold gas pressure for prismatic batteries may be between about 10PSIG and about 40 PSIG. In some embodiments, the threshold gas pressurefor prismatic batteries may be between about 20 PSIG and about 30 PSIG.In the case of cylindrical batteries, the threshold pressure may be muchhigher, such as up to 500 PSIG or more.

While the electrodes 12, 14 are shown substantially spaced apart in thefigures, in some embodiments the electrodes may be very close to oneanother or even compressed against one another with a separator 16 inbetween. Furthermore, although the figures may illustrate a singlepositive electrode 12 and a single negative electrode 14, batterysystems within the scope of the present disclosure may also include twoor more positive electrodes 12 and/or two or more negative electrodes14.

In some embodiments, features may be provided at an interface betweenelectrodes and the separator to aid in allowing gas bubbles to escapeupwards rather than become trapped at an electrode/separator interface.Some embodiments of such gas-escape structures may include geographicfeatures structured into a face of an electrode that create spacesbetween the separator and recessed regions of the electrode. Suchgeographic features may include linear shaped channels with longitudinaldirections typically (though not necessarily) extending substantiallyvertically relative to the electrolyte level.

Other gas-escape geographic structures may include regularly-spaced,arbitrarily-spaced, or randomly-spaced peaks and valleys or corrugations(preferably with a minimum of sharp “peaks” that may puncture theseparator and cause the electrodes to short-circuit) arranged such thatthe peaks contact the separator while the valleys create gas spaces. Asuitably connected network of such peaks and valleys may create windingor meandering pathways for gas bubbles to flow upwards between theelectrode and the separator.

As used herein, the term “gas-escape structures” may be defined in oneor more ways. In some embodiments, gas-escape structures may be definedin terms of a minimum, maximum, average or other aggregate measure of adistance between proximal (i.e., closer to a central plane of aflattened electrode) “low-points” and distal (i.e., furthest from acentral plane of a flattened electrode) “high-points.”

In some embodiments, an electrode may be said to include gas-escapestructures if a minimum or average distance between high points and lowpoints is at least 100 microns or more, 1 mm or more, or 5 mm or more inother embodiments. In some embodiments, an electrode may be said toinclude gas-escape structures if a minimum or average distance betweenhigh points and low points is at least ten times as large as the largestparticle of active material. In some embodiments, an electrode may besaid to include gas-escape structures if a minimum or average distancebetween high points and low points is at least 10% of the electrode’sthickness (where electrode thickness defined as the distance between ahighest-point on one electrode face to a high-point on the oppositeface.

In further embodiments, gas-escape structures may be provided as anadditional layer of material interposed between an electrode and aseparator. Alternatively or in addition, gas-escape structures may beprovided as structures formed in the separator. For example, channeled,corrugated, or net-shaped sheet structures may be interposed between anelectrode and a separator. In other examples, a separator may be formedwith folds, corrugations, or multiple layers with channels or otherelevated structures in order to create gas-escape spaces between anelectrode and the separator.

However, in some cases it may be desirable to minimize or substantiallyeliminate spaces or structures between an electrode and a separator inwhich gas bubbles may form, accumulate or travel, in which such aconfiguration may tend to cause gases to be forced through the separatorwhere they may interact with the counter-electrode. Such embodiments maybe particularly desirable so as to encourage direct recombination ofoxygen on a negative electrode. Arrangements in which an electrodecontacts a separator with a minimum of gas accumulation spaces betweenthe electrode and the separator (i.e., arrangements that do not includeany of the gas escape structures described herein) may be referred toherein as the electrode being in “uniform contact” with the separator.In an embodiment, for example, an electrode being in “uniform contact”with a separator refers to an arrangement wherein less than 0.1% of thevolume, and for some applications less than 0.01% of the volume, betweenthe electrode and the separate corresponds to spaces and/or structuresin which gas bubbles may form, accumulate or travel.

In some embodiments, a layer of porous inert conductive material may bepositioned between an electrode and a separator for the purpose ofrecombining gas that crosses the separator before it reaches theelectrode adjacent to it. Example inert materials may be “inert” in thatthey do not contain species that will tend to be electrochemicallyoxidized or reduced in the battery. For example, suitable inertmaterials may include carbon paper, carbon felt, inert metal meshes,perforated foils, foams, etc. Examples of such inert metals may includeany of the materials described as possible current collectors, such asnickel, titanium, copper, etc.

Additionally, the number and/or cumulative capacity of positiveelectrodes may be unequal to the number and/or total cumulative capacityof negative electrodes. For example, in some embodiments, a batterysystem may have a larger total cumulative negative electrode capacitythan total cumulative positive electrode capacity. On the other hand,some embodiments of a battery system may have a larger total cumulativepositive electrode capacity than total cumulative negative electrodecapacity. In various embodiments, providing a larger total capacity ofone electrode relative to its counter-electrode may limit theopportunity of the larger-capacity electrode to be driven to anover-charge, over-discharge, or polarity reversal condition at which itmay generate significant quantities of gas.

In some embodiments, a separator 16 material may be selected to have anincreased permeability to hydrogen and/or oxygen gas. In someembodiments, a separator 16 may be a multi-layered separator having oneor more hydrophobic gas transport layers and one or more hydrophilicion-transport separator layers. For example, a separator may comprise atleast one layer of hydrophobic material and at least one layer of ahydrophilic material, thereby allowing for both gas transfer and ion(electrolyte) transfer through the separator.

In some embodiments, portions of a separator material may be madehydrophobic or hydrophilic by treating or coating the separatormaterial. For example, in some cases a hydrophilic material may bepatterned with hydrophobic regions by a corona discharge treatment,laser treatment, chemical treatments, or others. Various hydrophobicand/or hydrophilic coatings and/or chemical treatments may also beapplied to a separator to impart corresponding characteristics to theseparator material in order to encourage gas transport across theseparator without substantially inhibiting ionic transport.

In some embodiments, as illustrated for example in FIG. 2 , direct gasrecombination may be aided by arranging a battery such that a portion ofat least one battery electrode extends above the electrolyte level 22into the gas headspace, thereby allowing gases to recombine byinteraction with the electrode (or electrodes) exposed to the gasheadspace. For example, in one embodiments a negative electrode 15 maybe sized so as to be physically larger than the positive electrode 12such that the negative electrode 15 extends into the headspace while thepositive electrode 12 remains submerged in the electrolyte 20.Alternatively, a positive electrode may be sized so as to be physicallylarger than the negative electrode such that the positive electrodeextends into the headspace while the negative electrode remainssubmerged in the electrolyte. In still other embodiments, a battery maybe configured such that both electrodes are partially flooded and extendat least partially into the headspace.

In some embodiments, a battery containing multiple positive electrodesand multiple negative electrodes may have one or more negativeelectrodes or one or more positive electrodes sized larger than theremaining electrodes such that the larger (positive or negative)electrode extends into the headspace while the remaining positive andnegative electrodes remain submerged in the electrolyte. In someembodiments, a portion of an electrode surface extending into theheadspace may be coated with a catalyst material selected to catalyzechemical or electrochemical hydrogen oxidation or oxygen reductionreactions.

In various embodiments, electrode stacks (i.e., an assembly of at leastone positive electrode, a separator and at least one negative electrode)may be arranged in various configurations such as “free-standing” and“compressed.” In a free-standing configuration, the positive andnegative electrodes may be spaced apart from the separator or in loosecontact with the separator. In a compressed configuration, theelectrodes may be in tight contact with the separator, where the tightcontact is maintained by a structure forcing the stack componentsagainst the separator(s). In some embodiments, a compressed electrodestack may be held in a compressed state by a spring shim or otherstructure interposed between a wall of a battery container (whethercylindrical or prismatic) and a portion of the battery stack. In someembodiments, a compression pressure may be applied by a batterycontainer structure configured to apply a compressive pressure to thebattery stack. For example, a prismatic or cylindrical battery containermay include compression bolts, screws, or other structures configuredfor applying a compressive pressure to the electrode stack. In variousembodiments, an applied compression pressure may be at least 1 PSI, 10PSI, 20 PSI, 25 PSI, or more. In some embodiments, the compressionpressure may be at least partially applied by expansion (or “swelling”of the battery electrodes) being restrained by a housing, can, or otherenclosure containing the electrodes and electrolyte.

Long Life Iron Electrodes

Without wishing to be bound by any particular theory, there may bediscussion herein of beliefs or understandings of underlying principlesrelating to the devices and methods disclosed herein. As also notedearlier, it is reiterated that inventors recognize that regardless ofthe ultimate correctness of any mechanistic explanation or hypothesis,an embodiment of the invention can nonetheless be operative and useful.

The presence of sulfide compounds such as iron sulfide in an ironelectrode may impact rate capability; the ability of the electrode to bedischarged at relatively high rates (e.g., see U.S. Pat. 9,577,298 andU.S. Pat. Application Publication 2015/086884, which are incorporatedherein by reference). Incorporation of electronically conductive sulfidecompounds such as iron sulfides in an iron negative electrode maycounter passivation caused by the discharge product, iron (II)hydroxide, which is an electronic insulator. Sulfide compounds arebelieved to maintain the electronic conductivity at the interfacesbetween adjacent iron particles and/or iron hydroxide particles and atinterfaces between iron (or iron hydroxide) particles and a currentcollector, allowing the discharge reaction to be sustained at highrates. Many electrolyte-soluble sulfide compounds may be used as asource-material for sulfide incorporation by an iron electrode.

Sulfide ions in an electrolyte solution (e.g., from a dissolved sulfideadditive or other sulfide source) may be incorporated into an ironelectrode during formation cycling. The exact chemical composition andsolid-state phase of sulfide compounds incorporated into an ironelectrode through formation cycling may vary and/or be unknown, but mayinclude one or more compounds of sulfur with iron, oxygen, and/orhydrogen (e.g., an iron sulfide phase, a solid solution of iron sulfideand iron hydroxide, a sub-oxide of iron sulfide, or others). Sulfideincorporated into an iron electrode during formation cycling may bereferred to herein as “incorporated sulfide.”

A sulfide-source material (e.g., a soluble metal sulfide) added into anas-made iron electrode may be referred to herein as a “sulfideadditive.” A sulfide-source material added to the electrolyte withoutbeing electrically connected to the iron electrode may be referred toherein as a “sulfide reservoir” or “reservoir sulfide.”

As used herein, the term “sulfide-source material” refers to anymaterial or substance that may deliver sulfide ions into electrolyte byany mechanism, including dissolution, electrochemical reduction oroxidation, mechanical injection, or otherwise. Various examples ofsulfide-containing or sulfur-containing materials are described herein,any of which may be a sulfide-source material within the broad meaningof the term. A “solid soluble sulfide-source material” is asulfide-source material in solid form that is at least slightly solublein an electrolyte solution.

The term “sulfide compound” refers to a chemical species comprisingsulfide ion(s) or a chemical species which may dissociate into sulfideion(s) upon dissolution in an electrolyte. A sulfide compound may referto an incorporated sulfide compound, an additive sulfide compound, asulfide-source material in a sulfide reservoir, or all of these. Theterm “sulfide ion” refers to S²⁻ or an ion comprising S²⁻. A sulfide ionmay be present in solid form as part of a solid compound (e.g., a solidionic compound). A sulfide ion may be a dissolved sulfide ion, such as asulfide ion dissolved in an electrolyte. The term “sulfide” may refer toa sulfide ion or a sulfide ion-containing compound. In some embodiments,the term “sulfide” refers to sulfide ion(s).

Incorporated sulfide may be irretrievably lost from the iron electrodeby multiple mechanisms under various conditions. Incorporated oradditive sulfide may be “lost” from the iron electrode by entering theelectrolyte solution via electrochemical reduction and/or bydissolution. Once in solution, sulfide may be “consumed” by beingirretrievably converted to (dissolved or solid) sulfur species such assulfites or sulfates.

The rate of electrochemical reduction of sulfide (incorporated orelectrically-connected additive) may substantially increase as theelectrode becomes more highly polarized. In other words, as thepotential of the iron electrode becomes more negative (i.e., as thenegative electrode is charged to higher states-of-charge), sulfide maybe increasingly lost from the iron electrode into the electrolytesolution. As sulfide is lost from the electrode and consumed withoutbeing replaced, performance of the battery will increasingly degrade.

A key element to producing a long-life Ni-Fe battery lies in the abilityto retain sulfide in the iron negative electrode throughout charging anddischarging cycles.

Sulfide may be added to an iron electrode at any stage. For example, asolid electrolyte-soluble sulfide-source material such as iron sulfide,bismuth sulfide, tin sulfide, zinc sulfide, manganese sulfide, asub-oxide of iron sulfide, or others, may be included as an additive inpowder or other particulate form into a negative electrode activematerial mix during fabrication of an as-made iron electrode.Additionally or alternatively, sulfide may be added to an iron electrodeduring formation processing by adding a sulfide-containing solution(e.g., sodium sulfide or potassium sulfide) into an electrolyte in whichformation cycling will occur. In further embodiments, sulfide may beadded to the electrolyte by placing a soluble solid sulfide-sourcematerial in the electrolyte, but electrically isolated from the ironelectrode.

The dissolved sulfide ions in the electrolyte will tend to be taken upby the iron electrode, typically by forming an in-situ incorporatedsulfide compound in the iron electrode through chemical reactions withspecies in the iron electrode such as iron, iron oxide, and/or ironhydroxide.

In some embodiments, a negative electrode may contain a metal sulfideadditive with large particles (e.g., 100 microns or greater) in additionto a metal sulfide additive of smaller particles (e.g., less than 100microns). In various embodiments, “small particles” may be those smallerthan 50 microns while “large particles” may be those larger than 50microns.

In some embodiments, sulfide may be provided in an iron electrode bycoating a current collector substrate with a sulfide material. Forexample, a current collector substrate may be pre-treated to form ametal sulfide on the current collector prior to pressing or otherwiseadhering active material onto the current collector.

Regardless of how sulfide is incorporated into the electrode,incorporated sulfide and sulfide additives, may leave (e.g., dissociate;e.g., dissolve) the iron electrode into the electrolyte solution bymultiple mechanisms.

For example, iron sulfide and other metal sulfides may beelectrochemically reduced to metallic iron and dissolved sulfide ionswhen charging the iron electrode. The rate of electrochemical reductionof iron sulfide may occur to some extent at all charging potentials, butincreases substantially at more negative potentials, especially duringover-charging of the negative electrode. Other incorporated sulfidespecies may also be electrochemically reduced to release sulfide ionsinto solution. Independent of charge or discharge potential, some amountof sulfide may simply dissolve in the electrolyte due to dissolutionthermodynamics. Both mechanisms release sulfide (S²⁻) ions intosolution.

Some sulfide ions in solution may migrate (e.g., by diffusion or othermechanisms) to the positive electrode where they will tend to beoxidized to produce various sulfur-containing species such as sulfitesor sulfates. Sulfide ions in solution may also react with oxygen bubblesor dissolved oxygen and may be similarly converted into sulfite orsulfate compounds. Oxygen may be produced by electrolysis side-reactionsat the positive electrode and may migrate across to the negativeelectrode in dissolved or gaseous form. The conversion of sulfide tosuch oxidized species is irreversible in that they cannot be easilyconverted back into sulfides in-situ within a battery. Because theconversion of sulfide to sulfites or sulfates is considered a one-wayprocess, sulfide that is converted to a sulfite or sulfate will bereferred to herein as having been “consumed.”

Sulfide that is lost from the iron electrode into solution and consumedmay be replaced in the iron electrode by additional sulfide ionsavailable in the electrolyte. For example, if un-incorporated additivesulfide or reservoir sulfide enters the electrolyte (e.g., is dissolvedor electrochemically reduced), that sulfide may become available forincorporation into the iron electrode by reacting with iron hydroxide orother species in the iron electrode. Therefore, if a rate of sulfideconsumption exceeds a rate at which unincorporated sulfide enterssolution, there will be a net loss of sulfide from the cell.

The net consumption of sulfide from the iron electrode may degradeperformance over time. Additionally, the produced sulfite or sulfatecompounds may themselves interfere with operation of the positiveelectrode, and in a sealed cell, sufficient oxidation of sulfide by thepositive electrode and/or by oxygen produced at the positive electrodemay lead to an imbalance in the states-of-charge of the positive andnegative electrodes.

When the quantity of sulfide in the iron electrode falls too low, theaccessible capacity of the iron electrode tends to drop precipitously,particularly for higher discharge rates, but eventually at allpracticable rates.

FIG. 3A illustrates discharge rate performance for three electrodescontaining different quantities of an iron sulfide additive. The chartof FIGS. 3 illustrates electrode utilization (discharge capacity perunit mass of active material) vs discharge rate (where a rate of “1 C”is the rate at which the electrode’s end-of-formation capacity may bedischarged in one hour). The electrode containing 0.1% iron sulfideadditive by weight of iron exhibits considerably lower utilization athigher discharging rates compared to slower discharge rates. Theelectrode containing 0.3% iron sulfide additive by weight of ironexhibits slightly lower utilization at a 1 C discharge rate compared toa C/2 discharge rate. By contrast, the electrode containing 1% ironsulfide additive by weight of iron exhibits about the same utilizationat discharge rates of C/5, C/2, and C.

FIG. 3B illustrates charge/discharge cycling of an iron electrodecontaining 0.3% iron sulfide additive by weight of iron. As can be seenat about 175 cycles, the discharge capacity drops, eventually falling toessentially zero. After the electrode failed, it was disassembled andthe inventors determined that the iron electrode had lost substantiallyall of the iron sulfide additive.

Preventing Net Loss of Incorporated Sulfide by Avoiding High Potentials

Without wishing to be bound by any particular theory, there may bediscussion herein of beliefs or understandings of underlying principlesrelating to the presence, composition, reactions and/or function ofincorporated sulfide, additive sulfide and/or reservoir sulfide inelectrochemical systems, and components thereof, disclosed herein. Thepresent inventors have discovered that the rate of electrochemicalsulfide compound, such as iron sulfide, reduction substantiallyincreases as the negative electrode becomes more highly polarized (i.e.,reaches more negative potentials). In particular, sulfide loss from theiron electrode appears to increase substantially when the electrodereaches potentials more negative than about -1.0 V. In other words, asthe potential of the iron electrode becomes more negative (i.e., as thenegative electrode is charged to higher states-of-charge), sulfide maybe increasingly lost and consumed, thereby increasingly degradingperformance of the battery.

FIG. 3C is a chart plotting the quantity of sulfide measured in theelectrolyte with a sulfide-ion-selective electrode vs. time for ironelectrodes (containing sulfide compound, such as iron sulfide) held atvarious specified electric potentials. As sulfide is lost from theelectrode, the quantity of sulfide detectable in the electrolyteincreases and is detected as a potential with the sulfide-selectiveelectrode calibrated to correlate detected voltages to sulfidequantities. As shown, when an electrode is held at -1.0 V, the rate ofincrease of sulfide in the electrolyte is very slow. As electrodes areheld at higher potentials, the slope of increasing sulfide in theelectrolyte becomes increasingly steep indicating rapidly rising ratesof sulfide leaving the electrode at more negative potentials.

In some embodiments, for example, minimizing the rate at which sulfideis permanently lost from an iron electrode and consumed is important toproducing an efficient, high-rate and long-life iron electrode. Stateddifferently, it is important to maintain a quantity of sulfide in theiron negative electrode in order to allow for an efficient, high-rate,long-life iron-electrode battery such as a nickel-iron battery.

In some embodiments, for example, sulfide ions in the electrolyte mayreact with iron hydroxide in the iron electrode to re-form orincorporate sulfide compounds, such as iron sulfides (e.g., FeS, Fe₃S₄or other iron sulfide compounds or solid solutions containing sulfide),in the iron electrode. As a result, the presence of available ironhydroxide in the iron electrode active material may substantially slowor even stop the net loss of sulfide from the electrode. This isbecause, as long as enough iron hydroxide is available, any dissolved orreduced sulfide proximate to a portion of iron hydroxide may be quicklyconverted back to sulfide compound(s) before being irretrievablyconverted to a sulfite or sulfate. In some embodiments, therefore, bymaintaining at least a minimum iron hydroxide reserve in the electrodeat all times, a desired minimum amount of sulfide compound(s) may beretained in the iron electrode for as long as sulfide remains available.

As used herein, the term “iron hydroxide” refers to anyhydroxide-containing iron compound, including Fe(OH)₂, Fe(OH)₃,anhydrous iron hydroxide such as FeO(OH), or hydrated forms of ironhydroxide of the form FeO(OH)•nH₂O (where n is a number of watermolecules in a hydrated iron hydroxide molecule), and any polymorphs ofthese or other forms.

When an iron electrode is charged, iron hydroxide is reduced to metalliciron. Therefore, in order to maintain a reserve of iron hydroxide,charging of the iron electrode must be prevented from converting all ofthe iron hydroxide to iron. In general, this may be achieved byretaining a reserve chargeable capacity in the negative electrode. Asused herein, the term “chargeable capacity” refers to a portion of theelectrode’s active material that may be converted from a “discharged”species to a “charged” species.

Stated differently, sulfide may be retained in an iron negativeelectrode by avoiding charging the iron negative electrode to a 100%state-of-charge, where the “state-of-charge” (or “SOC”) of an electrodeis defined as the percent of electrode active material that has beenconverted from a “discharged” species to a “charged” species. As usedherein, the SOC of a battery will be defined as the SOC of the electrodewith a lower total capacity compared to its counter-electrode. Thelower-capacity electrode in a battery will generally be referred to asthe “limiting electrode.”

In some embodiments sulfide may be retained in an iron negativeelectrode by never charging the iron electrode to an SOC greater than athreshold SOC. In various embodiments, a threshold SOC may be up toabout 99%. In some particular embodiments, a threshold SOC may bebetween about 50% and about 90%. In various specific embodiments, athreshold SOC may be about 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%,95%, or SOC values in between these.

In some embodiments, this may be achieved by coupling a positiveelectrode with an iron negative electrode that has a greater capacitythan the positive electrode. Assuming the overcharging of the positiveelectrode can be minimized, and assuming the positive and negativeelectrodes remain balanced (i.e., one coulomb of charge is added to thenegative electrode for each coulomb of charge added to the positiveelectrode and one coulomb of charge is removed from the negativeelectrode for each coulomb of charge removed from the positiveelectrode) this excess negative electrode capacity may ensure that thenegative electrode is never charged to a higher state-of-charge than thecapacity ratio of the electrodes. In other words, (with the aboveassumptions) if a battery has a negative-to-positive electrode capacityratio of 130% (i.e., the total negative electrode capacity is 130% ofthe total positive electrode capacity), then when all active material inthe positive electrode has been converted to a “charged” species (i.e.,starting with both electrodes at a 0% SOC, the positive electrode ischarged to a 100% SOC), only about 77% of the negative electrode activematerial will have been converted to a “charged” species.

In various embodiments, a battery with an iron negative electrode mayhave a negative-to-positive capacity ratio of between about 101% to 200%or more. In particular embodiments, a negative-to-positive capacityratio may be 110%, 115%, 120%, 125%, 130%, 135%, 140%, 145%, 150%, 155%,160%, 165%, 170%, 175%, 180%, 185%, 190%, 195%, 200%, 205%, 210%, 215%,220%, 225%, 230%, 235%, 240%, 245%, 250%, 255%, 260%, 265%, 270%, 275%,280%, 285%, 290%, 295%, 300% or more.

However, if the positive electrode is less efficient than the negativeiron electrode (i.e., if parasitic gas-producing side reactions stealsome charge from the positive electrode while charging), then duringeach charge cycle the negative electrode state-of-charge will tend toincrease more than the positive electrode state-of-charge. Eventually,the negative electrode will reach a 100% state-of-charge at which pointall of the iron active material will have been charged to metallic iron,leaving no iron hydroxide to recapture sulfide ions from theelectrolyte. Once the negative electrode reaches top-of-charge,subsequent cycles will tend to over-charge the negative electrode,further accelerating the net loss of sulfide from the negativeelectrode.

By incorporating gas recombination systems that return charge to thebattery electrodes (i.e., direct recombination and/or recombination withauxiliary electrodes connected to battery electrodes), anystate-of-charge imbalance created by gas evolution reactions may bereversed, thereby returning balance to the states-of-charge of theelectrodes and maintaining a desired capacity ratio between theelectrodes. This is explained in more detail in the followingparagraphs.

In some embodiments of a Nickel-Iron battery, the charging reactions aregiven by the equations:

-   Iron charging reaction: Fe(OH)₂ + 2e^(–) → Fe + 2OH^(–)-   Nickel charging reaction: Ni(OH)₂ + OH⁻ → NiOOH + H₂O + e⁻

The discharging reactions are the reverse of these. In variousembodiments, other charging and/or discharging reactions may occurinstead of or in addition to the above charging reactions. Whatever theexact reactions, the state-of-charge (SOC) of each electrode increasesas a charging reaction for that electrode occurs, and decreases as adischarging reaction for that electrode occurs.

However, when a gas evolution reaction occurs on an electrode, a quantumof charge that would otherwise have increased the SOC of that electrodeis instead diverted to the gas evolution reaction. If that same quantumof charge increases the SOC of the counter-electrode, then theelectrodes become un-balanced by an SOC difference equal to the divertedquantum of charge. As gas evolution reactions continue over time, themagnitude of such an SOC imbalance may increase.

An SOC imbalance may be reversed in one of two ways. Either the SOC ofthe electrode on which the gas evolution reaction occurred may beincreased by the quantum of charge diverted to gas generation, or theSOC of the counter-electrode may be decreased by that quantum of charge.

Conveniently, each molecule of gas that is evolved represents the exactquantum of charge diverted when evolving that gas molecule. Therefore,when that gas molecule is recombined at the counter-electrode (or anauxiliary electrode electrically connected to the counter-electrode),the recombination reaction will drive a discharge reaction in thecounter-electrode equal to the quantum of charge represented by the gasmolecule, thereby reversing the SOC imbalance caused by evolution ofthat gas molecule.

Even if the loss of sulfide compound(s) via electrochemical reduction isslowed as described above, the loss of sulfide to sulfite or sulfate maybe inevitable due to dissolution of incorporated sulfide as well as anyremaining electrochemical reduction. Some free sulfide ions in theelectrolyte may be converted to other species before encountering anyiron hydroxide even if “enough” iron hydroxide is generally available inthe electrode. This means that over time, a nickel-iron battery mayexperience a net loss of sulfide compounds from the iron electrodeunless a source of sulfide ions is provided to replace sulfide that islost and consumed.

Replacing Consumed Sulfide

In some embodiments, the consumption of sulfide may be addressed byperiodically adding a quantity of a make-up electrolyte solutioncontaining dissolved sulfide (e.g., a solution containing an alkalisulfide such as sodium sulfide). Alternatively, an entire volume ofelectrolyte may be periodically replaced in order to restore a desiredquantity of sulfide. However, both of these approaches may tend to causethe positive and negative electrode states-of-charge to be out ofbalance, eventually requiring a rebalancing procedure to return theelectrode states-of-charge to a desired balance.

One might assume that providing a large quantity of soluble sulfide inthe electrode or electrolyte would be adequate to replace any consumedsulfide. However, high concentrations of sulfide in the electrolyte maycause corrosion or other detrimental effects on the iron electrode,reducing its performance. In fact, the inventors have discovered thatdetrimental effects occur at dissolved sulfide concentrations far lowerthan previously understood.

Therefore, it is desirable to maintain an available quantity of sulfidein the electrolyte within an ideal “goldilocks” range of at least enoughavailable sulfide for the iron electrode to operate efficiently and toreplace that which is irretrievably consumed, but not so much dissolvedsulfide that corrosion or other detrimental effects occur. The inventorshave determined that the lower boundary of the desired sulfideconcentration range may exist below the name-plate detection limit(e.g., approximately 1 µM) of conventionally available in-situmeasurement techniques (e.g., the sulfide-selective electrode describedabove), and is therefore not necessarily accurately known. In someembodiments, the upper boundary of the desired sulfide concentration mayalso lie below the 1 µM detection limit.

The inventors have also determined that the minimum amount of sulfiderequired to efficiently operate an iron electrode (i.e., the lowerboundary of the desired range) is greater than the solubility limits (inalkaline electrolyte) of highly insoluble metal sulfides such as CuS,FeS₂ (pyrite), and Bi₂S₃. On the other hand, the inventors havediscovered that detrimental effects of sulfide are seen atconcentrations lower than the next-least-soluble metal sulfides (e.g.,ZnS, FeS, MnS). The inventors have further found an approximately threeorders-of-magnitude gap between these “highly insoluble” metal sulfidesand the next-least-soluble metal sulfides in alkaline electrolytes, atleast among commonly available and environmentally safe metal sulfides.Some metal sulfides with solubility limits within the desirable rangemay exist and may be used (e.g., CdS may have a solubility limit withinthe desired range). However, in addition to maintaining a sulfideconcentration within the desired range, it is also desirable to minimize(or preferably eliminate) the presence of environmentally unsafe heavymetals such as cadmium or lead within the battery.

The solubility limit of FeS in alkaline electrolyte has been measured tobe approximately 3 mM (3×10⁻³ moles/L) at about 25° C. The solubilitylimit of ZnS in alkaline electrolyte has been measured to be as high asapproximately 0.7 mM at about 25° C. Although the solubility limits ofCuS, FeS₂, and Bi₂S₃ in alkaline electrolyte are not known exactly, theyare believed to be less than 0.01 µM (1×10⁻⁸ moles/L), possibly lessthan 0.001 µM at about 25° C.

Therefore, in some embodiments, it may be desirable to maintain asulfide concentration in the electrolyte of at least a minimumconcentration, but no more than a maximum concentration, at least at anaverage operating temperature (e.g., at or about “room temperature”between about 20° C. +/- and about 30° C., and possibly between about 0°C. and about 60° C.). The ideal range of sulfide concentration for aparticular iron-electrode battery application may be determinedempirically, and may depend on factors such as a desired range ofcoulombic efficiency of the battery, a desired range of voltaicefficiency of the battery, an intended operating temperature range ofthe battery, a desired separator material, a desired counter-electrodecomposition, or other aspects of the battery’s construction or intendedapplication.

In some embodiments, the maximum sulfide concentration may be maintainedat less than a solubility limit of zinc sulfide or manganese sulfide,and the minimum sulfide concentration may be maintained at greater thana concentration of bismuth sulfide or copper sulfide. In someembodiments, the minimum maintained sulfide concentration may at leastabout 0.01 µM or at least about 0.1 µM. In some embodiments, the maximumsulfide concentration may be maintained at less than about 0.7 mM (700µM), less than about 0.1 mM (100 µM), or less than about 1 µM.

In various embodiments, the minimum desired concentration of sulfideions dissolved in the electrolyte may be at least about 0.01 µM, atleast about 0.05 µM, at least about 0.1 µM, at least about 0.15 µM, atleast about 0.2 µM, at least about 0.25 µM, at least about 0.3 µM, atleast about 0.35 µM, at least about 0.4 µM, at least about 0.45 µM, atleast about 0.5 µM, at least about 0.55 µM, at least about 0.6 µM, atleast about 0.65 µM, at least about 0.7 µM, at least about 0.75 µM, atleast about 0.8 µM, at least about 0.85 µM, at least about 0.9 µM, atleast about 0.95 µM, or at least about 1 µM.

In various embodiments, the maximum desired concentration of sulfideions dissolved in the electrolyte may be less than about 700 µM, lessthan about 650 µM, less than about 600 µM, less than about 550 µM, lessthan about 500 µM, less than about 450 µM, less than about 400 µM, lessthan about 350 µM, less than about 300 µM, less than about 250 µM, lessthan about 200 µM, less than about 150 µM, less than about 100 µM, lessthan about 90 µM, less than about 80 µM, less than about 70 µM, lessthan about 60 µM, less than about 50 µM, less than about 40 µM, lessthan about 30 µM, less than about 20 µM, less than about 10 µM, lessthan about 5 µM, less than about 1 µM, less than about 0.5 µM, less thanabout 0.1 µM, less than about 0.05 µM, or less than about 0.01 µM.

Therefore, simply providing a large supply of a sulfide source toreplace consumed sulfide may be counter-productive unless theconcentration of sulfide in solution at any given time can be maintainedwithin the desired range. The inventors have developed two broadapproaches to controlling the concentration of sulfide in theelectrolyte at any given time.

The first approach, broadly referred to herein as the “kinetic”approach, involves slowing down the rate at which sulfide is releasedinto the electrolyte so that, on average, sulfide is not released intothe electrolyte appreciably faster than it is consumed. The secondapproach, broadly referred to herein as the “thermodynamic” approachinvolves achieving a thermodynamic equilibrium at which theconcentration of sulfide is thermodynamically held approximately withina desired “goldilocks” range.

Kinetic Approaches to Controlling Sulfide Concentration

Various materials, structures, and methods may be used to control therate or timing of delivery of sulfide into the electrolyte of aniron-electrode cell in order to replace sulfide that is irretrievablyconsumed in the cell. Some methods may involve the periodic, timed, orevent-responsive release of sulfide. Other methods may involve treatingor handling sulfide-source materials so as to slow down the rate ofsulfide release into the electrolyte.

In order to further extend the usable life of a nickel-iron batteryrequiring minimal maintenance, a long-term reservoir of soluble sulfidemay be provided in the battery. As used herein, the term “sulfidereservoir” may refer to a source of sulfide ions other than an “additivesulfide” and an “incorporated sulfide,” both of which are located withinand electrically connected to the iron negative electrode. “Reservoirsulfide” may be located outside of but electrically connected to theiron electrode, located inside of but electrically isolated from ordisconnected from the iron electrode, or both located outside of andelectrically disconnected from the iron electrode. Sulfide may generallybe released from a sulfide reservoir into the electrolyte by chemicalreactions, electrochemical reactions, phase change reactions, and/orcontrolled mechanical actions (e.g., movement of a servo, piston, relay,or other electromechanical devices), or combinations of these or othermechanisms.

In some embodiments, a closed-loop automatic control system may beconfigured to detect a condition or an event directly or indirectlysuggesting a need for a sulfide addition to the electrolyte, and upondetecting the event or condition, delivering or releasing a quantity ofa sulfide source from a sulfide reservoir into the electrolyte. Forexample, in some embodiments a sulfide detector (e.g., a sulfideion-selective-electrode, an optical sulfide detector, or others) may bejoined to an automatic controller configured to periodically orcontinuously detect a sulfide concentration in the electrolyte with thesulfide detector. In response to detecting a sulfide concentration belowa threshold, the control system may activate an actuator device todeliver a sulfide-source material into the electrolyte. For example, theactuator may be a pump, syringe, or plunger configured to deliver aquantity of a solid or liquid sulfide-source material into theelectrolyte, for example, which may be the same amount every time or adifferent amount.

In other embodiments, an automatic control system may be configured todetect one or more events that may be indicative of a need for sulfidein the electrode. For example, an electronic controller may beconfigured to monitor cell performance and to operate an actuator todeliver a sulfide-source material to the electrolyte in response todetecting low-sulfide event. Example low-sulfide events may include adrop in coulombic efficiency greater than a threshold change, a decreasein discharge rate capability greater than a threshold amount, asubstantial period of over-charge (e.g., a fixed period of time, or apredetermined quantity of overcharge in coulombs), a change inelectrolyte conductivity greater than a threshold amount, or otherevents.

In some embodiments, low-sulfide events may be “detected” chemicallyand/or electrochemically in such a way as to chemically orelectrochemically trigger an automatic release of sulfide. In anembodiment, for example, the system is configured such that detection orcharacterization of a low-sulfide event is used as a triggering eventresulting in an “automatic release” of sulfide, for example, using aactive or passive system or method for releasing sulfide.

In various embodiments, an actuator may be configured to release ordeliver a consistent quantity of sulfide each time the actuator istriggered, or the actuator may be configured to release or deliver aquantity of sulfide in proportion to a quantitative measure of atriggering event.

In some embodiments, a sulfide reservoir may be configured to releasesulfide ions into the electrolyte at a slow rate in a location withinthe battery adjacent to the negative electrode such that a substantialportion of the released sulfide ions will reach the iron electrode toreplace consumed sulfide. In some embodiments, a sulfide-source materialfor a sulfide reservoir may comprise one or more soluble metal sulfidessuch as iron sulfide (e.g., FeS, FeS₂, Fe₃S₄ or other iron sulfidecompounds or combinations thereof), zinc sulfide, manganese sulfide,lead sulfide, nickel sulfide, tin sulfide, bismuth sulfide, coppersulfide (CuS, Cu₂S, or other copper sulfides), or cadmium sulfide,including any polymorphs of these, or combinations of these and/or othermetal sulfides. In some embodiments, a sulfide-source material for asulfide reservoir may include one or more sub-oxides of iron sulfide ofthe form FeS_(1-x)O_(x). In some embodiments, preferred materials for asulfide reservoir may comprise sparingly soluble metal sulfides, that ismetal sulfides that release no more than 10 milli-moles of sulfide ionsper liter of electrolyte at temperatures up to 70° C.

In some embodiments, a sulfide reservoir may be configured to have aslow rate of release of sulfide from the reservoir into the electrolyte.The rate of release of sulfide from a sulfide reservoir may be a rate ofdissolution if the reservoir is a solid sulfide source that releasessulfide by dissolution, a rate of electrochemical reduction if thereservoir is configured to release sulfide ions by electrochemicalreaction (e.g., by electrochemical reduction of a solid sulfide sourceelectrically connected to the negative electrode), a rate of injectionor release of a liquid sulfide source, and/or a rate of release and/ordissolution of a gaseous sulfur source (e.g., SO₂ or H2S).

A rate of dissolution of a solid sulfide reservoir in aqueous alkalinebattery electrolyte may be a function of surface area of the sulfidereservoir exposed to the electrolyte, dissolution kinetics of asulfide-source material, diffusion kinetics and/or dissolution kineticsof a barrier surrounding a sulfide-source material, a temperature of theelectrolyte, a solubility limit (saturation limit) of the sulfidereservoir material, and a rate at which sulfide is removed from theelectrolyte solution by absorption at the negative electrode or byirretrievable conversion to sulfite or sulfate, among other factors.

In some embodiments, a sulfide reservoir may be a “slow-release sulfidereservoir” in that they are configured to deliver sulfide ions to theelectrolyte at a rate slower than a natural rate of dissolution of thesame sulfide-source material placed directly in the electrolyte. Inother words, “slow-release” sulfide reservoirs may have a rate ofrelease of sulfide ions less than a natural dissolution rate of thesulfide-source material contained in the reservoir. Some embodiments ofslow-release sulfide reservoirs may include structures and materialsselected to dissolve and/or otherwise release sulfide ions atpredictably slow rates under conditions expected to be experienced bythe battery in operation. In some embodiments, the rate of sulfide ionrelease can be approximately matched with a rate of sulfide consumption(e.g., conversion to sulfate by oxygen or the positive electrode), suchthat an instantaneous sulfide concentration in the electrolyte at anygiven time or an average sulfide concentration over a period of time maybe maintained within a desired range.

In some embodiments, a sulfide-source material may be enclosed,surrounded, coated, or encapsulated within a barrier material selectedto slow the rate of dissolution of a sulfide source by slowing ordelaying electrolyte access to the sulfide-source material. In anembodiment, the terms enclosed, surrounded, coated, or encapsulated maybe used interchangeably.

In some embodiments, particles of a sulfide-source material may be atleast partially encapsulated in an encapsulation material that dissolvesmore slowly than the sulfide-source material. In various embodiments, anencapsulation material may be selected to electrically insulate anencapsulated sulfide-source material, to substantially increase anelectrical resistance of an encapsulated sulfide-source material, orboth. In general, it may be desirable for electrolyte to have at leastminimal access to the encapsulated material, such as via imperfections(e.g., pinholes or gaps) in an encapsulation coating, or by degradationof the encapsulation material in the electrolyte over time. If asulfide-source material is perfectly encapsulated inelectrolyte-impermeable material that does not degrade in theelectrolyte, then some other mechanism (e.g., mechanical abrasion,temperature changes, etc.) may be provided to allow the electrolyte toaccess and dissolve the sulfide-source material.

In various embodiments, encapsulation materials may include a metaloxide layer (e.g., bismuth oxide, copper oxide, aluminum oxide, zincoxide, or other metal oxide), a silicon oxide or silicon dioxide layer,silicone, or a polymer layer such as polyvinyl alcohol, polyethylene,polypropylene, rubbers (e.g., styrene butadiene rubber or others), ethylcellulose, methyl cellulose, latex, epoxide resins, polymethylpentene(PMP or TPX), polyethersulfones, sulfonated tetrafluoroethylene (e.g.,NAFION), other fluoropolymer copolymers (e.g., ACIPLEX, FLEMION, DOWEW,FUMAPEM F), other ionomer materials, or others. In some embodiments,encapsulation materials may comprise one or more phase-change materialsconfigured to undergo a phase change when an iron electrode experiencesan elevated temperature for sufficient time to melt the phase-changematerial. Such phase-change materials may include a paraffin,polyethylene glycol, solid hydrate, or a sugar alcohol tuned to meltand/or dissolve in electrolyte at a threshold temperature. In someembodiments, an encapsulation material may comprise two or more layersand/or two or more of these or other materials.

In some embodiments, sulfide-source material particles may be at leastpartially coated or encapsulated using any of various spray coatingprocesses, dip-coating processes, micro-encapsulation processes, orothers. For example, the paper “Coating of Metal Powders with Polymersin Supercritical Carbon Dioxide” by Glebof et. al., published in thejournal of Industrial & Engineering Chemistry Research in 2001 (DOI:10.1021/ie0100939) describes processes for coating metal particles(e.g., Al and Mg) and non-metal materials (e.g., fused silica) withpolymers, such as poly(vinylidene fluoride) and poly(4-vinylbiphenyl).U.S. Pat. 8486498 describes methods of coating metal powders withcurable compositions of cyanoacrylates. U.S. Pats. 4452861, 5328522,6416863, and 4994326 describe various methods of coating metal powderswith polymers and other coating materials so as to protect the particlesfrom chemical interactions prior to melting or otherwise removing thecoatings. Any of such methods may be adapted for coating orencapsulating sulfide-source material particles as described herein.

In some embodiments, encapsulated sulfide-source material particles maythen be included within an iron negative electrode. In some embodiments,a quantity (e.g., about 0.1% to 10% or more by weight of iron activematerial) of one or more sub-oxides of iron sulfide of the formFeS_(1-x)O_(x) may be included within an iron electrode active materialmix. Such sub-oxides may have a substantially slower dissolution ratethan iron sulfides or other sulfide-source materials, and may thereforerelease sulfide more slowly within the iron electrode.

In some embodiments, a sulfide reservoir may be provided as aslowly-dissolving composite solid submerged in the electrolyte. Asulfide reservoir may be made to dissolve more slowly by producing a“pill” of low surface area such as a single large sphere or prismaticsolid. In some embodiments, one or more slow-release sulfide “pills” maybe submerged in the electrolyte or otherwise positioned within a batterycontainer and joined to the electrolyte by an ionic pathway. Such aslow-release pill may comprise multiple alternating layers of asulfide-source material and one or more layers of a slowly-dissolvingencapsulation material such as those described above. In still otherembodiments a slow-release pill may be added to a battery as part of aperiodic maintenance procedure.

In some embodiments, the dissolution rate of slow-release pills or otherencapsulated sulfide-source materials may be determined based on amaterial and/or a thickness of an encapsulation material layer orlayers. In some embodiments, both relatively slowly-dissolving pills orencapsulated sulfide-source materials and faster-dissolving pills orencapsulated sulfide-source materials may be included within a batterycontainer. For example, a battery container may contain one pill withone or more layers of an encapsulation material configured to dissolveand expose an encapsulated sulfide-source material within a period ofbetween about one year and about five years. The same battery containermay also contain a pill with an encapsulation material layer (or layers)configured to dissolve and expose the encapsulated sulfide-sourcematerial within a period of between about five years and ten yearsand/or a pill with an encapsulation material layer (or layers)configured to dissolve in about 10 years to 20 years or more.

In some embodiments, a sulfide-source material (encapsulated or not) maybe retained within a shield structure configured to provide minimalfluid communication between the sulfide source and the electrolyte. Inone example, an electrolyte source material may be contained within asmall-diameter tube of electrolyte-impervious shield material (e.g.,stainless steel or an alkaline-electrolyte impervious polymer). Theinterior diameter of the tube may be selected so as to minimizediffusion of electrolyte and sulfide ions therethrough. For example,such a tube may have an interior diameter of between about 0.1 mm andabout 3 mm. Alternatively, a polymer may be extruded over asulfide-source material formed into a shape of a wire. Asulfide-source-filled tube may be coiled or otherwise shaped so as tofit within a battery container. Such units may be structured andpositioned within a battery container so as to be replaceable.

In some embodiments, a sulfide-source material may be contained within ashield material through which sulfide ions will very slowly diffuse. Forexample, a quantity of a sulfide-source material may be sealed into abag of Nafion or other ion-selective separator material. Nafion isgenerally described as excluding transport of anions like sulfide, but asmall amount of anion leakage still occurs. Therefore, the rate ofsulfide ion transport out of a sealed Nafion bag will tend to be veryslow. Other ion selective or microporous separator materials that mayallow slightly faster rates of sulfide release may also be used to formbagged sulfide structures that may be placed within a battery containersubmerged in the electrolyte or otherwise positioned so as to releasesulfide ions into the electrolyte.

In some embodiments, a sulfide-source material (encapsulated, bagged, ornot) may be positioned in a headspace within a battery container andjoined to the electrolyte by an ionic pathway such as those describedherein with reference to auxiliary electrodes. In particular, anelectrolyte-end of an ionic pathway such as a wicking structure, a tube,or other ionic pathway conduit may be positioned adjacent to a negativeelectrode (as shown for example by 222 in FIG. 8 ).

In some embodiments, a sulfide reservoir positioned within the headspacemay be electrically connected to the negative electrode. In someparticular embodiments, an oxygen reduction auxiliary electrode may beformulated to include a quantity of a sulfide-source material such asiron sulfide. A sulfide-containing auxiliary electrode may be positionedin the headspace and joined to the electrolyte by an ionic pathway. Thelimited rate of ion transfer via the ionic pathway may slow the rate ofrelease of sulfide ions from the auxiliary electrode into electrolytebelow the natural rate of dissolution of the iron sulfide if it weresubmerged directly in the electrolyte.

A sulfide-containing auxiliary electrode may be connected to a negativeelectrode by an electronic component such as a diode selected to limitthe potential of the auxiliary electrode, thereby causing the sulfidereservoir material to also be exposed to a limited negative potential,thereby limiting electrochemical reduction of sulfide from such asulfide-containing auxiliary electrode.

In some embodiments, a sulfide-source material may be positioned incontact with the electrolyte (e.g., submerged or joined by an ionicpathway) and electrically connected to the positive electrode via acombination of conductors and electronic components. While the sulfidesource is held at a positive potential of the positive electrode, thesulfide will be protected from dissolution in the alkaline electrolyte.The electrical connection joining the sulfide source to the positiveelectrode may then be periodically opened, leaving the sulfide sourceexposed to dissolution in the electrolyte for a period of time. In someembodiments, the electrical connection may be opened by operating anelectromechanical switch that may be operated by a battery chargecontroller or battery management system. Such a controller may beconfigured to open the switch on a pre-determined schedule, or based onoperating characteristics of the battery, such as when a decrease indischarge capacity or discharge rate capability is detected. In someembodiments, the electrical connection may be periodically opened basedon other factors, such as when the electrolyte temperature exceeds orfalls below a pre-determined temperature. This may be achieved with atemperature-sensitive switch that opens when exposed to a particulartemperature range, or with a controller and a temperature sensor. Instill other embodiments, the electrical connection may include a lowfrequency electronic oscillator (e.g., a relaxation oscillator or avoltage-controlled oscillator) configured to intermittently expose thesulfide reservoir to zero or negative potentials at which the sulfidesource may dissolve.

In some embodiments, a sulfide reservoir may be made in the form of asheet of a sulfide-source material encapsulated or bound with a bindermaterial. For example, an electrically conductive or non-conductivesheet of material may be made by blade-coating (or “tape-casting”) aconductive or non-conductive substrate with a mixture of sulfide-sourcematerial particles and a binder and/or a plasticizer. Example bindersmay include polymers, alcohols (e.g., polyvinyl alcohol), rubbers (e.g.,styrene butadiene rubber or others), ethyl cellulose, methyl cellulose,latex, epoxide resins, or any other encapsulating material as describedherein.

In some embodiments, a layer of a sulfide-source material may besandwiched between two (or more) insulative polymer layers and pressedand/or heated to form a non-conductive sulfide reservoir sheet. Anelectrically non-conductive sulfide reservoir sheet may advantageouslyrelease sulfide due to dissolution alone (i.e., without beingelectrochemically reduced).

In other embodiments, a conductive sulfide reservoir sheet may be formedfrom a sulfide-source material by hot-pressing, blade-coating,sintering, or other processes. A conductive sulfide reservoir sheet incontact with an iron electrode may advantageously release sulfide byboth dissolution and electrochemical reduction.

A sulfide reservoir sheet may be placed between two layers of aniron-electrode or between two iron electrodes which may be electricallyconnected to a common current-collector and/or a common tab so as to actas a single electrode.

In some embodiments, sulfide-absorbing additives may be included withinan iron electrode, which may assist by absorbing and re-releasingsulfide. Alternatively, sulfide absorbing additives may be included nearor in a separator on a negative-electrode side of the separator. Suchadditives may include copper, copper hydroxide, cadmium, cadmiumhydroxide, iron hydroxide, or others.

Thermodynamic Approaches to Controlling Sulfide Concentration

In further embodiments, systems and methods for controlling a sulfideconcentration may rely on one or more thermodynamic mechanisms. In thiscontext, “thermodynamic mechanisms” for controlling sulfideconcentration refers to factors affecting the tendency of asulfide-source material to dissolve in a liquid electrolyte. Forexample, in some embodiments, the electrolyte of a battery may containan additive that depresses the effective solubility of thesulfide-source material below the solubility limit of the sulfide-sourcematerial in an identical electrolyte at the same temperature without theadditive. One example of such an additive is described as a “counter-ionsource material.” Without wishing to be bound by any particular theory,it is believed that the use of a “counter-ion source material” todepress the concentration of sulfide in an electrolyte makes use of thethermodynamic mechanism known as the “common ion effect.” However, othermechanisms may also be at work. Nonetheless, regardless of the ultimatecorrectness of any mechanistic explanation or hypothesis, embodiments ofthe systems and methods described below can nonetheless be operative anduseful.

The common-ion effect is a consequence of the equilibrium interaction ofions in a saturated solution. A solution is “saturated” with aparticular solute when no more of the solute will dissolve in thesolvent (at a particular temperature). The concentration of the solutethat can be dissolved in a saturated solution is a quantity referred toas the “solubility limit” which is determined by the “solubilityproduct” of the solute in solution. The solubility product, typicallydenoted by the symbol “K_(sp)” is the product of the concentrations ofthe ions resulting from dissolution of a dissolvable solid (e.g., asalt). The solubility product of many specific solids in particularelectrolyte solutions have been determined and are generally availablein reference tables, and the solubility of any solid material in aparticular electrolyte solution may be determined empirically.

Upon dissolving in a liquid, a solid solute will typically separate intotwo or more ions. The “solubility product” of a compound of the for AxByis equal to the concentration of the first ion, (“A” in thisexplanation) to the exponent x, multiplied by the concentration of thesecond ion (“B″) to the exponent y.”). Stated in aqueous chemicalnotation (using brackets to denote “concentration of”):

The common ion effect states that, because Ksp must be a constant (for aparticular solvent at a particular temperature), if two solidscontaining the same ion are dissolved in the same solvent, the effectivesolubility of both solids will be controlled by the ion that is “common”to both solids.

For example, when sodium chloride (NaCl) dissolves in a solvent (e.g.,water), a sodium cation (Na⁺) and a chloride anion (Cl⁻) are released.Therefore, the solubility product of sodium chloride (NaCl) is equal tothe product of the sodium and chloride ions:

When copper chloride (CuCl) is dissolved, a copper cation (Cu⁺) and achloride anion (Cl⁻) are released. Therefore, the solubility product ofcopper chloride (CuCl) is equal to the product of the potassium andchloride ions:

Therefore, if both NaCl and CuCl are dissolved in the same solution, theamount of both solids that can dissolve will be limited by theconcentration of the chloride (Cl⁻) in solution. In a solutioncontaining a substantial concentration of dissolved NaCl, theconcentration of chloride will be determined by the quantity of NaClthat has dissolved. If solid CuCl is added to that solution, the amountof the solid CuCl that will dissolve will be limited by the existingconcentration of chloride in solution because the product of [Cu+] and[Cl-] must remain constant (i.e., equal to the Ksp for CuCl). As aresult, the concentration of the copper cation (Cu⁺) in the solutionwill remain very low. The ability to control the depression of CuClsolubility by the addition of a particular quantity of dissolved NaCl ismade simpler by the fact that the K_(sp) of NaCl is multiple orders ofmagnitude greater than the K_(sp) of CuCl, meaning that a very smallamount of NaCl is needed to depress the concentration of dissolvedcopper ions in solution.

If the Ksp values of both solids in the solvent (e.g., electrolyte) areknown, then a quantity of dissolved NaCl needed to depress solubility ofCuCl to a specific concentration of copper ions may be calculated. Ifthe Ksp values are not known for a particular solvent, such as for acomplex electrolyte solution containing multiple other dissolvedspecies, then a quantity of dissolved NaCl needed to depress solubilityof CuCl to a specific concentration of copper ions may be determinedempirically.

This same mechanism may be used to depress the concentration of sulfideions (S²⁻) in an electrolyte solution to a concentration substantiallylower than the solubility limit (or “saturation limit”) of a selectedsparingly soluble solid sulfide compound. For example, a low sulfideconcentration may be maintained by selecting a solid sparingly solublesulfide compound, identifying the “counter-ion” other than sulfide thatis released upon dissolving the sulfide compound, and identifying asecond soluble compound (i.e., a “counter-ion source material”) thatreleases the same counter-ion as the selected sparingly soluble sulfidecompound.

As used herein, a “sparingly soluble” sulfide compound is a materialwith a solubility limit corresponding to release of no more than 10 mM(1×10⁻² mol/L) of sulfide ions. The use of a sparingly soluble sulfidecompound as a target for selecting a counter-ion material maybeneficially allow for more granular control of a target depressedsulfide concentration in the electrolyte.

As used herein, the term “counter-ion” refers to any ion other thansulfide released into an electrolyte solution by dissolution of aselected sparingly soluble sulfide compound, whether or not the selectedsparingly soluble sulfide compound is initially present as a solid inthe cell. In the case of soluble metal sulfides, the counter-ion maytypically be a metal cation. In some cases, a solid sparingly solublesulfide-source material may release two or more counter-ions, any one ofwhich may be a counter-ion for the purposes of identifying a counter-ionsource material.

In some embodiments, the selected sparingly soluble sulfide compound maybe the same sulfide-source material used as a sulfide additive in anelectrode or a sulfide reservoir. Alternatively, the selected sparinglysoluble sulfide compound may be a different metal sulfide or non-metalsulfide compound. In some embodiments, the selected sparingly solublesulfide compound need not even be included in the electrode.

In one example, the selected sparingly soluble sulfide compound is zincsulfide, but the sulfide additive included in an iron electrode is ironsulfide (which has a higher Ksp than zinc sulfide). In this case, thecounter-ion is Zn²⁺ as that is the counter-ion released upon dissolutionof zinc sulfide. The presence of enough Zn²⁺ in the electrolyte may tendto cause any sulfide ions in excess of the solubility product of [Zn²⁺]and [S²⁻] (e.g., from dissolved iron sulfide or other sulfide-sourcematerial with a solubility limit greater than ZnS) to precipitate out ofsolution as zinc sulfide. In this way, the effective concentration ofdissolved sulfide ions from any sulfide-source material (solid orliquid) may be depressed by including a counter-ion that will producethe selected sparingly soluble sulfide compound.

Preferred counter-ion source materials may have much higher solubilitylimits (e.g., two, three, four, or more orders of magnitude greater)than a selected sparingly soluble sulfide compound as well as anysulfide compounds present in an iron electrode or in a sulfidereservoir. A quantity of the counter-ion source material may bedissolved in the electrolyte in sufficient quantity to depress thesolubility of the sulfide-source to or below a desired sulfideconcentration.

In a complex electrolyte, such as an alkaline solution containing asubstantial quantity of hydroxide ions, a counter-ion may be inequilibrium balance with one or more ions formed by reaction with thehydroxide ions. For example, in the case of zinc sulfide, the immediatecounter-ion is Zn²⁺, which may spontaneously react with hydroxide ionsto form Zn(OH)₄ ²⁻ (zincate). The Zn²⁺ ions and zincate ions may tend toachieve an equilibrium, maintaining a consistent (though not necessarilyequal) concentration of each. The formation of such additional ionicspecies may also increase the solubility of some materials in alkalinesolutions depending on the concentration of hydroxide and/or otherdissolved species.

For simplicity of explanation, formation of such additional ionicspecies and equilibrium states will not be described for allelectrolytes and solids. For at least these reasons, it may be difficultto predict or determine the Ksp value for a particular solid in acomplex electrolyte solution. In such cases, empirical methods may beused to determine the quantity of a counter-ion source material neededto depress solubility of sulfide ions to a desired concentration. Therelationship between concentration of various counter-ion sourcematerials and resulting sulfide concentrations in a particularelectrolyte may also be modeled based on theoretical and/orempirically-derived information.

For example, a known quantity of a counter-ion source material (orsolution) may be added to an electrolyte solution, a sulfide-sourcematerial may be added to the solution and allowed to dissolve until itreaches its solubility limit (depressed by the counter-ion) and theconcentration of sulfide may be measured, such as with asulfide-selective electrode as described above. After performing such amethod for several known quantities of the counter-ion source material,a model curve may be developed. In some embodiments, the model curve maybe extrapolated to un-measured points, and the extrapolated curve may beused to estimate expected sulfide concentrations even below thedetection limit of the sulfide-selective electrode or other technique(s)used to measure sulfide concentration. Alternatively, a known quantityof a counter-ion source (or solution) may be added to the electrolyte ofan assembled battery, which may then be cycled to determine whethercycling performance (e.g., based on efficiency metrics, discharge rates,discharge capacities, etc.) is consistent with a desired concentrationof sulfide for a desired period of time.

As described elsewhere herein, solid soluble sulfide-source materialsmay include various metal sulfides, such as bismuth sulfide, ironsulfide, iron disulfide, iron-copper sulfide, zinc sulfide, manganesesulfide, tin sulfide, copper sulfide, cadmium sulfide, silver sulfide,titanium disulfide, lead sulfide, molybdenum sulfide, nickel sulfide,antimony sulfide, including polymorphs of these. Solid solublesulfide-source materials may also include one or more sub-oxides of ametal sulfide, or a solid solution of a metal sulfide and an oxide orhydroxide. Solid soluble sulfide-source materials may include mineralssuch as a sulfosalt mineral, which is a salt of a metal (e.g., Cu, Pb,Ag, Fe, Hg, Zn, V), a semi-metal (e.g., As, Sb, Bi, Ge) and sulfur.Example sulfosalts include Pyrargyrite Ag₃SbS₃ and Tennantite Cu₁₂As₄S₁₃In some cases (e.g., depending on an electrolyte composition or otherfactors), a solid soluble sulfide source may include, non-metal sulfidecompound, such as dimethylsulfide, carbon disulfide, or others.

Some example sulfide-source materials and corresponding counter-ionsource materials are shown below in Table 1 in no particular order.

TABLE 1 Example Sulfide Compounds and Counter-ion Source MaterialsSulfide-Source Material Example Counter-Ion Source Materials bismuthsulfide bismuth oxide, bismuth hydroxide, bismuth metal magnesiumsulfide magnesium oxide, magnesium hydroxide, magnesium metal zincsulfide zinc oxide, zinc hydroxide, zinc metal tin sulfide tin oxide,tin hydroxide, tin metal manganese sulfide manganese oxide, manganesehydroxide, manganese metal copper sulfide copper oxide, copperhydroxide, copper metal lead sulfide lead oxide, lead hydroxide, leadmetal nickel sulfide nickel oxide, nickel hydroxide, nickel metalpyrargyrite (a sulfosalt mineral of Ag₃SbS₃) silver oxide, silverhydroxide, silver metal, antimony oxide, antimony hydroxide, antimonymetal titanium disulfide titanium oxide, titanium hydroxide, titaniummetal cadmium sulfide cadmium oxide, cadmium hydroxide, cadmium metal

The examples of sulfide-source materials and counter-ion sourcematerials shown above in Table 1 are simplified in the sense thatcounter-ions for particular metal sulfide sources are shown as the metalcation(s) produced by dissociation of the metal sulfide. However, asdescribed above, the counter-ion need not necessarily be the same cationas that released by dissolution of a metal sulfide additive included inan as-made electrode or a sulfide-source material in a sulfidereservoir. Therefore, in many embodiments, various combinations ofsulfide-source materials and counter-ion source materials havingdifferent cations may be used. For example, electrolyte-solublehydroxides or oxides of metals such as Ni, Mn, Cu, Zn, Pb, Cd, Sn orothers may be used as counter-ion source materials for depressingsolubility of different metal sulfides such as FeS, NiS, ZnS, MnS, orothers.

A counter-ion source material will generally release one or moreadditional ions or materials other than the selected sulfidecounter-ion. In some embodiments, the counter-ion source material may beselected to release an ion that has minimal (or no) deleterious effectson any other battery components, such as a negative electrode, apositive electrode, a separator, an auxiliary electrode, an electrolyte,or other cell components. For example, counter-ion source materials mayinclude oxides or hydroxides of a counter-ion metal. When dissolved,such oxides or hydroxides may simply produce water, hydroxide ions,dissolved oxygen, or other species. In some cases, a pure metal may beinsoluble in an electrolyte but may nonetheless corrode into an oxide orhydroxide that may be more soluble.

A counter-ion source material (also referred to herein as a “counter-ionadditive”) may be included in an electrolyte by any method as desired.For example, a solid soluble counter-ion source material may be firstdissolved in a separate liquid that may be mixed with an electrolyte, ora solid soluble counter-ion source material may be dissolved directly inan electrolyte solution. In still other examples, a solution containinga counter-ion may be synthesized directly as a liquid, such as by one ormore electrochemical processes. In other embodiments, a solublecounter-ion source material may be incorporated into an as-made ironelectrode as an additive (e.g., as a pore-former) which may dissolveafter electrolyte is added to the battery.

In some embodiments, this technique can be used to produce an alkalineelectrochemical cell comprising an iron electrode and a solid solublesulfide-source (i.e., additive sulfide, incorporated sulfide, orreservoir sulfide), wherein the concentration of sulfide in theelectrolyte is less than the solubility limit of a solidelectrolyte-soluble sulfide-source material present in the cell aseither a sulfide additive or a sulfide reservoir. For example, in someembodiments, a sulfide concentration may be held below 90% of thesolubility limit of the solid sulfide-source material in an electrolyteof the same composition without a counter-ion additive (and at the sametemperature). In various other embodiments, a sulfide concentration maybe held below 80%, 70%, 60%, 50%, 40%, 30%, 20%, 10%, 5%, 1%, 0.5%,0.1%, 0.05%, or 0.01% or less of an undepressed solubility limit of thesolid sulfide-source material under the same conditions.

In other examples, a battery may comprise an iron electrode, anelectrolyte, and a solid electrolyte-soluble sulfide-source material, inthe iron electrode as an additive, as incorporated sulfide, or otherwisein the electrolyte as a sulfide reservoir, that dissolves into sulfideand a first counter-ion, the electrolyte also containing a dissolvedquantity of a second counter-ion (which may be the same as the firstcounter-ion contained in the soluble sulfide-source material or adifferent counter-ion as described above) in a concentration sufficientto hold the concentration of sulfide in the electrolyte below apredetermined concentration at a desired temperature. For example, sucha battery may contain a counter-ion concentration sufficient to depressthe concentration of sulfide below 1×10⁻³ mol/L, 1×10⁻⁴ mol/L, 1×10⁻⁵mol/L, 1×10⁻⁶ mol/L, or 1×10⁻⁷ mol/l at 25° C. In some embodiments, theiron electrode may be un-formed (that is, in a state prior to formationcycling) when the sulfide concentration is below the above-describedthresholds. In some embodiments, however, system conditions are employedto provide for [S²⁻] < 1 µM.

In some embodiments, a battery may comprise an iron electrode, anelectrolyte, and a solid electrolyte-soluble sulfide-source material (inthe iron electrode as an additive, as incorporated sulfide or otherwisein the electrolyte as a sulfide reservoir), and further comprising anelectrolyte additive selected to depress a solubility limit of thesulfide-source material. In various embodiments, the electrolyteadditive (i.e., dissolved counter-ion material) may be present in aconcentration of at least 1 mM, 2 mM, 3 mM, 4 mM, 5 mM, 6 mM, 7 mM, 8mM, 9 mM, 10 mM, 20 mM, 30 mM, 40 mM, 50 mM, 60 mM, 70 mM, 80 mM, 90 mM,100 mM, 110 mM, 120 mM, 130 mM, 140 mM, 150 mM, 160 mM, 170 mM, 180 mM,190 mM, 200 mM, or more. In some embodiments, the electrolyte additivemay comprise one or more of nickel oxide, nickel hydroxide, manganeseoxide, manganese hydroxide, copper oxide, copper hydroxide, zinc oxide,zinc hydroxide, lead oxide, lead hydroxide, cadmium oxide, cadmiumhydroxide, tin oxide, tin hydroxide, or oxides and/or hydroxides ofother metals that form sparingly soluble metal sulfides. Theabove-described concentrations of dissolved counter-ion material mayrepresent concentrations of dissolved metal species regardless of ionicoxidation state (e.g., an electrolyte may contain a concentration ofdissolved ZnO in the range above, which may be present in the form ofZn²⁺, zincate, or other ionic species).

In various embodiments, the battery electrolyte may have a sulfideconcentration between a minimum sulfide concentration and a maximumconcentration, wherein the minimum sulfide concentration in theelectrolyte is at least about 0.01 µM, at least about 0.05 µM, at leastabout 0.1 µM, at least about 0.15 µM, at least about 0.2 µM, at leastabout 0.25 µM, at least about 0.3 µM, at least about 0.35 µM, at leastabout 0.4 µM, at least about 0.45 µM, at least about 0.5 µM, at leastabout 0.55 µM, at least about 0.6 µM, at least about 0.65 µM, at leastabout 0.7 µM, at least about 0.75 µM, at least about 0.8 µM, at leastabout 0.85 µM, at least about 0.9 µM, at least about 0.95 µM, or atleast about 1 µM; and the maximum sulfide concentration is less thanabout 700 µM, no more than about 650 µM, no more than about 600 µM, nomore than about 550 µM, no more than about 500 µM, no more than about450 µM, no more than about 400 µM, no more than about 350 µM, no morethan about 300 µM, no more than about 250 µM, no more than about 200 µM,no more than about 150 µM, no more than about 100 µM, no more than about90 µM, no more than about 80 µM, no more than about 70 µM, no more thanabout 60 µM, no more than about 50 µM, no more than about 40 µM, no morethan about 30 µM, no more than about 20 µM, no more than about 10 µM, nomore than about 5 µM, no more than about 1 µM, no more than about 0.5µM, no more than about 0.1 µM, no more than about 0.05 µM, or no morethan about 0.01 µM. In some particular embodiments, the system isconfigured to provide a concentration of dissolved sulfide ions ([S²⁻])in a range of about 0.1 µM ≤ [S²⁻] ≤ 700 µM. In some embodiments, thesystem is configured to provide a concentration of dissolved sulfideions ([S²⁻]) in a range of about 0.1 µM ≤ [S²⁻] ≤ 300 µM.

Such a battery may also contain a concentration of a sulfide counter-ionthat exceeds a concentration corresponding to the dissolvedsulfide-source material. For example, even after partial dissolution ofa sulfide-source material and consumption of some dissolved sulfide, theoriginal quantity of the sulfide-source material may be determined bymeasuring a total sulfur content of the battery including sulfites andsulfates.

Thus in some embodiments, a battery may comprise an iron electrode, anelectrolyte, and a solid electrolyte-soluble sulfide-source material (inthe iron electrode as an additive, as incorporated sulfide and/orotherwise in the electrolyte as a sulfide reservoir), wherein theelectrolyte contains a concentration of counter-ions capable of forminga solid sulfide compound with sulfide ions, and wherein a ratio of totalmoles of dissolved and solid counter-ions to total moles of solid anddissolved sulfur in the battery exceeds a stoichiometric ratio ofcounter-ions to sulfur in the sulfide-source material. In someembodiments, the sulfide-source material may be a metal sulfide otherthan iron sulfide and/or the counter-ion may be a metal other than iron.

Because the solubility limit of a sulfide-source material will tend tovary with temperature, a quantity of a counter-ion source material to beincluded in an electrolyte may be selected so as to depress solubilityof sulfide to a desired degree at one or more operating temperatures,such as a minimum operating temperature, a maximum operatingtemperature, an average operating temperature, or any other desireddesign point. Therefore, any of the values above may be selected for aparticular combination of sulfide-source material, counter-ion,counter-ion source material, and operating temperature. The relationshipbetween concentration of various sulfide-source materials, counter-ionconcentrations, and temperature of a particular electrolyte may bedetermined empirically and/or modeled based on theoretical and/orempirically-derived information.

In various embodiments, kinetic approaches can be combined withthermodynamic approaches. For example, the solubility limit of a solublesulfide-source material in one or more of the various sulfide reservoirsdescribed herein may be depressed by including a suitable counter-ionadditive in the electrolyte.

Over time, consumption of sulfide (as described above) will decrease theconcentration of sulfide in the electrolyte, thereby allowing more ofthe sulfide-source material to dissolve. Dissolution of additionalsulfide-source material will release more of the counter-ion (in caseswhere the sulfide-source material is the same as the counter-ionmaterial), which will further depress the solubility of thesulfide-source material. Therefore, it may still be desirable to limitthe rete of dissolution of a solid sulfide-source material and/or a rateof consumption of sulfide by various methods as described herein.

In some embodiments, including a separator with a low diffusioncoefficient may slow the rate at which sulfide is consumed at thepositive electrode. Similarly, a separator that allows a minimum ofoxygen bubbles to cross over the separator to interact with the negativeelectrode may further slow the rate of sulfide consumption. As describedabove, in various embodiments, a separator may be selected to have adiffusion coefficient of less than about 1×10⁻⁶ cm²/s.

Gas Recombination Devices

In some embodiments, a battery system may include one or morerecombination devices configured to perform gas recombination inaddition to or instead of relying on direct gas recombination on batteryelectrode active material.

In some embodiments, a recombination device may include a substratesupporting a catalyst. In some embodiments, a recombination device maybe physically separated from a battery electrode such that a substrateof the recombination device is not in physical contact with and/orphysically contiguous with a substrate supporting a battery electrode.

FIG. 4 illustrates a cross-section of one example of a recombinationdevice structure 400. The device 400 of FIG. 4 may include a hydrophobicgas diffusion layer 402 made of a hydrophobic material configured toallow gas to diffuse therethrough while substantially limiting orpreventing liquid from passing through. The gas diffusion layer 402 maycomprise expanded PTFE or other hydrophobic gas-diffusion materials. Insome embodiments, the device may also include a current collector layer404 made of an electrically conductive material such as a metal (e.g.,nickel, copper, iron), coated metals (e.g., nickel-coated ornickel-plated steel) or inert material coated with a metal. In someembodiments, the device may include a catalyst layer 406 made of aconductive material (e.g., carbon, graphite, a metal or a metal-coatednon-metal) on which a catalyst may be coated. In some embodiments, thecurrent collector layer and catalyst layer may be combined into a singlelayer. In some embodiments, the layers of a recombination device may beadhered or compressed together so as to form a unitary structure.

In various embodiments, porous substrates may be advantageous inproviding a large surface area on which reactions may occur. In someembodiments, an electrically conductive substrate material may bedesired, while in other embodiments a non-conductive substrate may beused. Examples of conductive substrates may include graphite, woven ornon-woven carbon felt, carbon fiber structures, ordered carbonstructures such as graphene and other allotropes of carbon or othercarbon-based structures capable of supporting a catalyst.

In some embodiments, solid, porous, fibrous, or other metal structuresmay be used as conductive substrates for recombination devices. Forexample, recombination devices may include one or more catalysts coatedonto metal foam, metal mesh, metal sheets, metal foils, or sinteredstructures. In some cases, a substrate of one metal may be coated with adifferent metal to improve conductivity or to reduce undesiredreactivity. Substrate metals may include nickel, iron, steel, copper,silver, nickel ferrite, cobalt ferrite, spinel-coated materials, orothers.

In some embodiments, a substrate material supporting a recombinationcatalyst may also support a battery electrode active material. Forexample, in some embodiments a first region of a conductive substratemay be coated with a battery active material mixture, and a secondregion of the same substrate may be coated with a gas recombinationcatalyst. In various embodiments, the first and second regions may beoverlapping or non-overlapping.

In other embodiments, a recombination device substrate may include anon-conductive material such as a polymer, ceramic, or compositematerial coated with a conductive metal to produce a conductivesubstrate with a non-conductive core. Such conductive metals may becoated onto non-conductive materials by electrodeposition, spraying,dipping, mechanical compression, or other processes.

In various embodiments, one or more catalysts may be coated onto asubstrate by various processes such as electrodeposition, galvanicdeposition, spraying, dipping, sintering, mechanical compression,electroless plating, sputtering, vapor deposition, hydrothermalsynthesis, or other processes resulting in at least a portion of asubstrate surface area carrying the catalyst material. In someembodiments, suitable processes used to apply a catalyst to a substratemay be chosen based on characteristics of the substrate and/orcatalyst(s) selected. Examples of specific catalysts are describedbelow.

For a gas to react on a recombination device, the gas must reach therecombination device via some gas pathway. Gas generated at an electrodemay be dissolved into the electrolyte and may then move through theelectrolyte by diffusion. Generated gas may also form bubbles which maymove through the electrolyte by buoyancy or other forces. Once gasbubbles float to the top of the electrolyte, the gas will tend to bereleased from the electrolyte into the gas headspace above theelectrolyte level in the battery container.

Therefore, in some cases, creating a gas pathway may simply involveproviding a recombination device with access to the headspace above theelectrolyte level, either by placing at least a portion of therecombination device directly in the headspace or joining therecombination device with the headspace by a gas-transfer conduit. Invarious embodiments, a gas-transfer conduit may comprise various hollowstructures such as tubing, hoses, pipe or channels, hydrophobicstructures or other structures through which gas may flow. In othercases, gas pathways may be formed on or in objects below the electrolytesurface with hydrophobic materials or hollow conduits allowing gaswithin the electrolyte or the headspace to travel to a desired locationbelow the electrolyte surface. In various embodiments, gas pathways maybe formed with hydrophobic materials or hollow conduits providing gascommunication from the headspace to locations below the electrolytelevel.

In addition to a gas pathway, some recombination devices may alsoutilize electrical connections to other elements of a battery such asone or more battery electrodes, a battery container, a currentcollector, or other battery structures. A recombination device may beoptimally configured for one or more desired chemical or electrochemicalreactions. For example, recombination device structures, connections, orchemical compositions may be configured with a preference for one ormore reactions. Additionally, electrical connections between therecombination device and other battery elements may be configured with apreference for one or more reactions. Similarly, ionic connectionsbetween the recombination device, the electrolyte, the batteryelectrodes, or other battery elements may be configured with apreference for one or more reactions.

Ionic connections (also referred to herein as “ionic pathways”) mayinclude any pathway, conduit, or medium through which ions may beexchanged between a recombination device and one or more other elementsof a battery (e.g., electrolyte and/or a battery electrode). In someembodiments, a recombination device may be ionically connected to theelectrolyte and/or a battery electrode by an ionically conductive path.For example, a recombination device may be in ionic communication withthe electrolyte by a fluidic/ionic pathway through which ions may beconducted. In some embodiments, a recombination device may be secured ata fixed point above the electrolyte level within the battery container.In such embodiments, an ionic pathway may be a wicking structure such asone or more capillary tubes, a section of a hydrophilic wicking material(e.g., a hydrophilic separator material as described herein), asubmerged section of a recombination device substrate, or other wickingstructures. In other embodiments, an ionic pathway may comprise one ormore conduits (e.g., tubes, pipes, hoses, channels, or other hollowstructures) through which electrolyte may flow under the force of apump, gravity, thermal siphon, diffusion, or other driving force.

FIG. 5 illustrates a battery system 10 comprising a battery container 18containing an electrolyte 20, a positive electrode 12, a negativeelectrode 14, and a separator 16. FIG. 5 also illustrates multiplepossible positions for a recombination device within or relative to thebattery container. A recombination device in any of the illustratedpositions may be configured to support one or more of oxygen reductionreactions, hydrogen oxidation reactions, hydrogen/oxygen combustion, orother reactions.

FIG. 5 illustrates multiple alternative locations inside and outside ofthe battery container 18 at which a recombination device may be located,along with corresponding ionic pathways and gas pathways (electronicconnections are omitted in FIG. 5 for simplicity, but may include any ofthe electrical connection configurations described herein). For example,recombination device 52 is shown positioned entirely in a head-space 24above the electrolyte level 22 with an ionic connection 54 to theelectrolyte 20.

Another example embodiment is represented by recombination device 56positioned such that a portion of the recombination device 56 extendsbelow the electrolyte level 22 in order to provide an ionicallyconductive path. In such an arrangement, an upper portion of therecombination device 56 extending into a headspace 24 may provide a gaspathway.

In some embodiments, the recombination device 56 may be rigidly securedto the battery container 18 (or another structure) at a fixed positionrelative to the electrolyte level 22. In other embodiments, therecombination device 56 may be configured with a buoyant element sizedto cause the recombination device 56 to float on the electrolyte surface22, thereby providing consistent access to both the gas headspace 24 andthe liquid electrolyte 20 even if the electrolyte level 22 changesrelative to the battery container 18 due to the conversion ofelectrolyte volume to gas, due to tipping the battery container, orother changes in the electrolyte level.

In some embodiments, a recombination device may be positioned entirelybelow the electrolyte level. FIG. 5 shows recombination device 58submerged below the electrolyte level 22 and including a gas pathwayelement 60. The gas pathway element 60 may include a hollow conduit suchas one or more tubes or a hydrophobic element such as a sheet of ahydrophobic material such as polytetrafluoroethylene (PTFE) or otherhydrophobic material. An alternative submerged recombination device 63may extend longitudinally in the depth direction of the electrolyte.Such a recombination device 63 may include a gas pathway element 61extending alongside the recombination device and providing a gas pathwayto a side of the recombination device. In some embodiments arecombination device 63 may be formed as layers of gas pathway materialand auxiliary active materials deposited on an interior wall of thebattery container. Alternatively or in addition, a recombination device63 may be positioned at a center of a cylindrically-wound electrodestack in a cylindrical battery.

FIG. 5 also shows recombination device 62 positioned outside of thebattery container 18. The recombination device 62 is joined to theheadspace 24 by a gas pathway conduit 64 extending between therecombination device 62 and the headspace 24 of the battery container18. Gas may travel through the gas pathway conduit 64 by diffusion orother motive force. The recombination device 62 may be ionicallyconnected to the battery 10 by an electrolyte conduit 66 extendingbetween the recombination device 62 and the electrolyte 22 within thebattery container 18. In various embodiments, gas pathway conduits andionic pathway (electrolyte) conduits may comprise any structuresdescribed herein, such as hollow tubes, pipes, hoses, channels,conduits, hydrophobic or hydrophilic elements (respectively), or otherstructures.

Recombination devices may also be located in a battery based on aposition relative to a battery electrode. For example, a recombinationdevice may be pressed, welded, or otherwise attached to a portion of abattery electrode such as at a top, bottom, side, or back surface. Insome embodiments, a recombination device may be separated from a batteryelectrode by a layer of ion permeable separator material or by a layerof an ion-impermeable material. In some embodiments, a recombinationdevice may be formed as part of a battery electrode structure. In otherembodiments, a recombination device may be welded or otherwisemechanically and electrically joined to a battery electrode such as bymechanical fasteners, clamping pressure, press-fitting, compressionjoining, or other methods. In some embodiments, a recombination devicemay be sandwiched between two or more battery electrodes and may beseparated from one or both by one or more ion-permeable orion-impermeable separators.

Auxiliary Electrodes

An “auxiliary electrode” is generally a recombination device configuredprimarily for supporting electrochemical gas recombination reactions. Anauxiliary electrode may typically be electrically connected (orconnect-able) to a battery electrode or other structure or elementimparting an electric potential to the auxiliary electrode. An auxiliaryelectrode may be connected to a battery electrode by an electricalconductor, which may include any combination of wires, printed circuitboard traces, flex connectors, electronic components (e.g., resistors,diodes, switches, capacitors, etc.), or other components capable ofcompleting an electrical connection between the auxiliary electrode andthe battery electrode or otherwise imparting an electric potential onthe auxiliary electrode.

FIG. 5 illustrates a battery system 100 with an auxiliary electrode 110electrically connected to a positive battery electrode 12 and ionicallyconnected to the electrolyte 20 by an ionic pathway 120 and directlyexposed to the headspace 24. Electrically connecting the auxiliaryelectrode 110 to the positive battery electrode 12 will expose theauxiliary electrode 110 to the positive electric potential of thepositive electrode 12, which will vary depending on the state-of-chargeand active material(s) of the positive electrode. For example, duringnormal operation a nickel positive electrode may typically be cycledbetween a half-cell voltage of about 0.1 V and about 0.6 V (depending onthe oxygen evolution potential of a particular nickel electrodecomposition and operating temperature).

In some embodiments, an auxiliary electrode 110 connected to thepositive electrode 12 may advantageously be provided with anelectrocatalyst for hydrogen gas oxidation at or below the potential ofthe positive electrode 12. In operation, such an auxiliary electrode 110may oxidize hydrogen gas as the gas reaches the electrocatalyst surface.Soluble species produced by the hydrogen oxidation reaction may bereturned to the electrolyte via the ionic connection 112. In variousembodiments, the auxiliary electrode 110 may be located in any of thepositions discussed above with reference to FIG. 5 or any other suitableposition and may include any of the structures described herein forproviding ionic and gas pathways.

If the potential applied to the positive electrode while charging thebattery is sufficiently large, then an auxiliary electrode 110 connectedto the positive battery electrode 12 may undesirably generate oxygenwhile the battery is charged and/or while standing at open circuitpotential. In order to limit or prevent such oxygen generation at theauxiliary electrode 110 during charging, the electrical connection 114between the positive electrode 12 and the auxiliary electrode 110 may bemade via an electronic component 120 configured to hold the auxiliaryelectrode 110 to an electric potential below the oxygen-generatingpotential for the electrocatalyst present on the auxiliary electrode 110surface.

In some embodiments, an electronic component 120 may be configured toprevent unwanted electric current flow between the auxiliary electrode110 and the positive electrode 12. For example, it may be desirable tolimit or prevent current flow between a battery electrode and anauxiliary electrode when such current would tend to cause a gasevolution reaction at an auxiliary electrode. Electronic components 120suitable for such purposes may include resistors, transistors, diodes,semiconductor devices, rectifiers, switches, or other electrical,electronic, or electromechanical devices.

In some embodiments, an electronic component 120 between an auxiliaryelectrode 110 and a positive electrode 12 may be configured to make theelectric potential of the auxiliary electrode 110 less positive than thepositive battery electrode at least during battery charging. In someembodiments, an electronic component 120 between an auxiliary electrode110 and a positive electrode 12 may also make the electric potential ofthe auxiliary electrode 110 less positive than the positive batteryelectrode during discharging or when the battery 100 is at open-circuit.

In other embodiments, an electronic component 120 may be configured toopen the circuit between the auxiliary electrode 120 and the positiveelectrode 12 during charging. For example, in some embodiments, theelectronic component 120 may include a switch or a relay which may becontrolled by a battery management system or other controller to preventcurrent flow between the positive electrode 12 and the auxiliaryelectrode 110 during charging of the battery. In some embodiments, anauxiliary electrode current may be monitored during various stages ofbattery operation. Higher auxiliary electrode currents may indicate arate of gas recombination and/or gas generation at the auxiliaryelectrode. Based on such information, a switch between the auxiliaryelectrode and the positive electrode may be opened or closed in order toprevent unwanted gas generation.

In other embodiments, an electronic component 120 may be configured toprevent or limit current flow in one or both directions. For example, insome embodiments, the electronic component 120 may include a diode(e.g., a Schottky diode, silicon diode, or others) configured to preventcurrent flow from the auxiliary electrode 110 to the positive electrode12 but allow current flow from the positive electrode 12 to theauxiliary electrode 110 (where a direction of current flow is oppositethe flow of electrons). In other embodiments, a resistor may be used tolimit current flowing in both directions. In further embodiments,combinations of two or more electronic components 120 may be used. Forexample, in some embodiments a diode may be used to substantiallyprevent current flow from the positive electrode 12 to the auxiliaryelectrode 110 and a resistor may be used to limit current flow from theauxiliary electrode 110 to the positive electrode 12. In furtherembodiments, an electronic component 120 may include an integratedcircuit configured to prevent the flow of unwanted currents.

In some embodiments, one or more electrical components, such as diodes,switches, or other components, may be used to prevent an auxiliaryelectrode electrically connected to the positive electrode fromdecreasing the potential of the positive electrode to potentials atwhich damage to the electrode may occur. For example, many nickelpositive electrodes contain cobalt compounds included to enhanceconductivity of the electrode. Such cobalt compounds may be mosteffective when in a particular form (e.g., cobalt oxyhydroxide). If thepotential of any portion of a nickel positive electrode falls below 0volts (vs MMO), the beneficial cobalt compounds may be reduced, whichmay cause temporary or permanent damage to the nickel electrode. At thesame time, an iron negative electrode may produce substantial quantitiesof hydrogen gas even at an apparent “bottom-of-charge” state (i.e., atan SOC of 0%). If excess hydrogen is oxidized on an auxiliary electrodeconnected to a nickel electrode at or near 0% SOC, the potential of thenickel electrode may be pulled low enough to reduce the cobalt anddamage the electrode.

Therefore, in some embodiments, one or more electrical components, suchas diodes, switches, or other components, may be used to stop currentflow between an auxiliary hydrogen oxidation electrode and a positivenickel electrode when the potential of the positive electrode fallsbelow a threshold half-cell voltage. In some embodiments, the thresholdpositive half-cell voltage may be about 0.01 V, 0.02 V, 0.03 V, 0.04 V,0.05V, 0.06 V, 0.07 V, 0.08 V, 0.09 V, or 0.1 V.

In some embodiments, a nickel electrode connected to an auxiliaryhydrogen oxidation electrode may be protected from undesirably lowpotentials by connecting the auxiliary electrode to the nickel electrodevia an electronic component that will prevent current flow between thepositive electrode and the auxiliary electrode if the potential betweenthem is less than an absolute value sum of the hydrogen oxidationpotential and the threshold voltage specified above. Thus, for example,if the hydrogen oxidation potential is taken to be -0.928 V vs MMO and adesired threshold is 0.05 V, then in order to protect the nickelelectrode, the electronic component may prevent current flow if thepotential difference across the electronic component is less than 0.978V. In some embodiments, the electronic components may be one or morediodes with a forward-pass voltage approximately equal to the absolutevalue sum of the hydrogen oxidation potential and the threshold minimumnickel half-cell voltage. Alternatively, the electronic component may bea switch or a combination of components arranged to achievesubstantially the same result.

In other embodiments, it may be similarly desirable to prevent apotential of a negative electrode from becoming too positive as a resultof oxygen reduction reactions occurring at an oxygen reduction auxiliaryelectrode. In such embodiments, an electrical component connecting theoxygen reduction auxiliary electrode to the negative electrode may beconfigured to prevent the negative electrode from reaching a potentialmore positive than about - 0.1 V.

FIG. 7 illustrates a battery system 200 with an auxiliary electrode 210electrically connected to a negative battery electrode 14 via aconductor 224 and an electronic component 220. Electrically connectingthe auxiliary electrode 210 to the negative battery electrode 14 willexpose the auxiliary electrode 210 to the electric potential of thenegative electrode 14, which will vary depending on the active materialand state-of-charge of the negative electrode.

The auxiliary electrode 210 is also shown ionically connected to theelectrolyte via a wicking ionic pathway 222, however any other ionicpathway may also be used as described herein. In some embodiments, anauxiliary electrode 210 connected to the negative electrode mayadvantageously be provided with an electrocatalyst for oxygen gasreduction at or above (i.e., less negative than) the potential of thenegative electrode 14. In operation, such an auxiliary electrode 210 mayreduce oxygen gas as the gas reaches the electrocatalyst surface.

An oxygen reduction auxiliary electrode connected to the negativeelectrode will tend to depolarize the negative electrode while oxygengas is being reduced, making the negative electrode’s potential lessnegative during time periods when oxygen gas is present to be reduced.Oxygen gas tends to be produced by a nickel hydroxide positive electrodewhen the positive electrode is at a high state-of-charge. Therefore,oxygen will tend to be produced during the same stages of operation whenthe presence of oxygen to be reduced by an oxygen auxiliary electrode ismost beneficial to retaining sulfide in a negative iron electrode. As aresult, a nickel-iron battery enjoys unique benefits from the use of anoxygen reduction auxiliary electrode connected to an iron negativeelectrode.

If the potential applied to the negative electrode 14 while charging thebattery is sufficiently negative, then an auxiliary electrode 210connected to the negative battery electrode 14 may undesirably generatehydrogen while the charging current is applied. In order to limit orprevent such hydrogen generation at the auxiliary electrode 210 duringcharging, the electrical connection between the negative electrode 14and the auxiliary electrode 210 may be made via an electronic component220 configured to hold the auxiliary electrode 210 to an electricpotential less negative than the hydrogen-generating potential for theelectrocatalyst present on the auxiliary electrode 210 surface.Alternatively, an electronic component 220 may be configured to preventunwanted electric current flow between the auxiliary electrode 210 andthe negative electrode 14. Electronic components 220 suitable for suchpurposes may include resistors, diodes, transistors, semiconductordevices, rectifiers, switches, or other electrical, electronic, orelectromechanical devices.

In some embodiments, an electronic component 220 between an auxiliaryelectrode 210 and a negative electrode 14 may be configured to make theelectric potential of the auxiliary electrode 210 less negative than thepotential of the negative electrode at least during battery charging. Insome embodiments, an electronic component 220 between an auxiliaryelectrode 210 and a negative electrode 14 may also make the electricpotential of the auxiliary electrode 210 less negative than thepotential of the negative electrode during discharging or when thebattery is at open-circuit.

In other embodiments, an electronic component 220 may be configured toopen the circuit between the auxiliary electrode 210 and the negativeelectrode during charging. For example, in some embodiments, theelectronic component 220 may include a switch or a relay which may becontrolled by a battery management system or other controller to preventcurrent flow between the negative electrode 14 and the auxiliaryelectrode 210 during charging of the battery. In some embodiments, anauxiliary electrode current may be monitored during various stages ofbattery operation. Higher auxiliary electrode currents may indicate arate of gas recombination and/or gas generation at the auxiliaryelectrode. Based on such information, a switch between the auxiliaryelectrode and the negative electrode may be opened or closed in order toprevent unwanted gas generation.

In other embodiments, an electronic component 220 may be configured toprevent or limit current flow in one or both directions. For example, insome embodiments, the electronic component 220 may include a diode(e.g., a Schottky diode, silicon diode, or others) configured to allowcurrent to flow from the auxiliary electrode 210 to the negativeelectrode 14 but prevent current from flowing from the negativeelectrode 14 to the auxiliary electrode 210 (where the direction ofcurrent flow is opposite the direction of electron flow). In otherembodiments, a resistor may be used to limit current flowing in bothdirections. In further embodiments, combinations of two or moreelectronic components 220 may be used. In further embodiments, anelectronic component 220 may include an integrated circuit configured toprevent the flow of unwanted currents.

As illustrated in FIG. 8 , some embodiments of a battery system may beconfigured with both a first auxiliary electrode 110 connected to apositive electrode 12 as described with reference to FIG. 6 and a secondauxiliary electrode 210 connected to a negative electrode 14 asdescribed with reference to FIG. 7 . In operation, such a system may bewell-suited to independently recombining hydrogen with the firstauxiliary electrode 110 and oxygen with the second auxiliary electrode210.

In some embodiments, when a battery contains two auxiliary electrodes,one connected to each battery electrode, the two auxiliary electrodesmay be configured to share gas pathway and/or ionic pathway structures.FIG. 9 and FIG. 10 illustrate one example of a dual auxiliary electrodestructure with a common gas pathway.

FIG. 9 illustrates a transverse cross-section of a dual auxiliaryelectrode device 900 comprising a first auxiliary electrode stack havinga gas-diffusion layer 902, a current collector layer 904 and a catalystlayer 906 and a second auxiliary electrode stack having a gas-diffusionlayer 912, a current collector layer 914 and a catalyst layer 916. Thefirst and second auxiliary electrode stacks may be arranged such thattheir gas diffusion layers are nearest each other, and a space betweenmay be filled with a gas pathway structure 922. The gas pathwaystructure may be any of the gas pathway structures described herein,such as an open conduit, a hydrophobic element, or other structure.

In some embodiments, the first and second auxiliary electrode stacks maybe surrounded by a sealing structure 920. The sealing structure may bean electrolyte-impervious material such as a polymer that may beover-molded, adhered, welded, or otherwise integrally sealed with theauxiliary electrode stack edges so as to prevent leakage of electrolyteinto the gas pathway in between the auxiliary electrode stacks. As shownin the longitudinal cross-section of FIG. 10 , the sealing structure maysurround the dual auxiliary electrode structure on three sides (e.g.,vertical sides and bottom).

In some embodiments, the catalyst layers 906 and 916 may be exposed,allowing the dual auxiliary electrode structure 900 to be submerged inthe electrolyte, leaving a top cap portion 924 exposed to the headspacein the battery container. In some embodiments, the cap portion 924 maybe covered with a gas pathway structure such as a hydrophobic element.

In alternative embodiments, the dual auxiliary electrode structure 900may be configured to be positioned in the headspace and joined to theelectrolyte by an ionic pathway. For example, hydrophilic wickingelements, electrolyte conduits, or other ionic pathway structures may besecured to the catalyst layers 906, 916 allowing electrolyte tocommunicate ions between the electrolyte and each of the auxiliaryelectrode stacks.

Alternatively, the auxiliary electrode stacks may be reversed such thatthe gas diffusion layers 912 are positioned on the outside of the stacksand therefore on the outside of the structure, while the central space922 may be filled with electrolyte and/or an ionic pathway structure.Such a structure may be placed in a headspace, allowing the exposed gasdiffusion layers access to gas in the headspace and joining the centralspace 922 to the electrolyte by an ionic pathway structure. In otherembodiments, such a structure may be submerged in the electrolyte.Hydrophobic or other gas pathway elements may be provided to allow eachof the gas diffusion layers to access gas in the headspace.

As used herein, a gas recombination auxiliary electrode’s “specificactivity” is a measure of the mole quantity of gas that may berecombined (hydrogen oxidized, or oxygen reduced) per unit of time(e.g., moles per second). This metric may be affected by multiplefactors such as a catalytic activity of a catalyst carried on theauxiliary electrode, a quantity of the catalyst and possibly physicaldimensions of the auxiliary electrode.

In some embodiments, in a battery with both an auxiliary oxygenreduction electrode and an auxiliary hydrogen oxidation electrode, theauxiliary oxygen reduction electrode may have a smaller specificactivity than the hydrogen oxidation electrode if the battery isconfigured to allow for substantial direct oxygen reduction on thebattery’s negative electrode. In other words, an auxiliary electrodespecific activity ratio (i.e., a ratio of the specific activity of anauxiliary oxygen electrode to the specific activity of an auxiliaryhydrogen electrode) may be less than 100%. In various embodiments, theauxiliary electrode specific activity ratio may be less than about 95%,less than about 90%, less than about 80%, less than about 70%, less thanabout 60%, less than about 50%, less than about 40%, less than about30%, less than about 20%, less than about 10%, less than about 5%, orless than about 1%.

FIG. 11 illustrates a battery system 300 with an auxiliary electrode 310electrically connected to both a positive battery electrode 12 and anegative battery electrode 14. In some embodiments, a system 300 such asthat shown in FIG. 11 may include switches or other electroniccomponents 320, 322 between the auxiliary electrode 310 and a positiveelectrode 12 as well as a negative electrode 14. Such electroniccomponents 320, 322 may be connected by any suitable conductors 324,326. Such a system 300 may be configured to selectively cause theauxiliary electrode 310 to recombine hydrogen by allowing current toflow between the positive electrode 12 and the auxiliary electrode 310or to recombine oxygen by allowing current to flow between the negativeelectrode 14 and the auxiliary electrode 310.

Nearly all catalysts suitable for catalyzing hydrogen oxidationreactions may also catalyze oxygen reduction reactions to some degree.In general, such catalysts may include platinum group metals (e.g., Pt,Pd, Ru, Rh, Os, Ir) or other metals (e.g., Ag, Au, Cu, Re, Hg), whichmay be used individually or in various combinations. In someembodiments, the nickel-aluminum alloy known as Raney nickel (otherwiseknown as “skeletal catalyst” or “sponge-metal catalyst”) may be used aseither a hydrogen oxidation catalyst or an oxygen reduction catalyst. Insome embodiments, Raney nickel doped with or coated with one or moreother metals (e.g., platinum group metals or other metals) may be usedto catalyze hydrogen oxidation reductions and/or oxygen reductionreactions. For example, Raney nickel doped with Mo, Fe, Ti, W, Cr, Cu,and Co may be used to catalyze hydrogen oxidation reactions. (e.g., see“Nickel supported on nitrogen-doped carbon nanotubes as hydrogenoxidation reaction catalyst in alkaline electrolyte” by Zhongbin Zhuanget. al., published in Nature Communications, Jan. 14, 2016.) The Zhuanget. al. paper also describes nickel supported on carbon nanotubes andnickel supported on nitrogen-doped carbon nanotubes as hydrogenoxidation catalysts.

Some catalyst materials are more suitable for oxygen reduction than forhydrogen oxidation. For example, various iron-containing materials maybe used as catalysts for oxygen reduction, including metallic iron,maghemite (Fe₂O₃), magnetite (Fe₃O₄), iron sulfide, iron hydroxide, orothers. Various silver-containing metals such as metallic silver, silvernitrate, silver oxides, or others may be used for oxygen reduction. Insome embodiments, oxygen reduction catalysts may be or include almostany metal. In some embodiments, an oxygen reduction catalyst may includea conductive carbon material such as carbon felts, graphite, carbonpaper, carbon black, or other forms of carbon.

In some embodiments, a hydrogen auxiliary electrode may include ahydrogen-absorbing material, such as a metal hydride, in addition to orin place of a hydrogen oxidation catalyst. An auxiliary electrodecontaining a metal hydride may absorb hydrogen gas as the gas isgenerated during charging of a battery. During discharging, such a metalhydride-containing auxiliary electrode may be electrically connected tothe negative battery electrode (e.g, via an electronic component such asa switch or a diode), thereby causing discharging of the metal hydrideauxiliary electrode. Alternatively, a recombination device disconnectedfrom either battery electrode may include a metal hydride. In such acase, absorbed hydrogen may be recombined with any available oxygen bycombustion.

FIG. 12 illustrates a battery system with a hydrogen/oxygen combustionelement. Hydrogen/oxygen combustion may be catalyzed by variousmaterials with optimum combustion catalytic activity at varioustemperatures. For example, “Catalytic Combustion of Hydrogen...” by M.Haruta and H. Sano, published in the International Journal of HydrogenEnergy Vol. 6, No. 6, pp. 601~08, in 1981 describes catalytic activityfor various combustion catalysts at various temperatures.

In some embodiments, a system such as that illustrated in FIG. 12 mayinclude a heating element coupled to the combustion recombinationdevice. In some embodiments, a combustion recombination device may beprovided in addition to one or more auxiliary electrodes or otherfeatures for encouraging direct recombination of gasses on batteryelectrodes.

Although the invention has been disclosed in the context of certainembodiments and examples, only the claims define the invention which mayinclude embodiments or examples beyond the specifically disclosedembodiments to other alternative embodiments and/or uses of theinvention and obvious modifications and equivalents thereof.

STATEMENTS REGARDING INCORPORATION BY REFERENCE AND VARIATIONS

The terms and expressions which have been employed herein are used asterms of description and not of limitation, and there is no intention inthe use of such terms and expressions of excluding any equivalents ofthe features shown and described or portions thereof, but it isrecognized that various modifications are possible within the scope ofthe invention claimed. Thus, it should be understood that although thepresent invention has been specifically disclosed by preferredembodiments, exemplary embodiments and optional features, modificationand variation of the concepts herein disclosed may be resorted to bythose skilled in the art, and that such modifications and variations areconsidered to be within the scope of this invention as defined by theappended claims. The specific embodiments provided herein are examplesof useful embodiments of the present invention and it will be apparentto one skilled in the art that the present invention may be carried outusing a large number of variations of the devices, device components,methods steps set forth in the present description. As will be obviousto one of skill in the art, methods and devices useful for the presentmethods can include a large number of optional composition andprocessing elements and steps.

As used herein and in the appended claims, the singular forms “a”, “an”,and “the” include plural reference unless the context clearly dictatesotherwise. Thus, for example, reference to “a cell” includes a pluralityof such cells and equivalents thereof known to those skilled in the art.As well, the terms “a” (or “an”), “one or more” and “at least one” canbe used interchangeably herein. It is also to be noted that the terms“comprising”, “including”, and “having” can be used interchangeably. Theexpression “of any of claims XX-YY” (wherein XX and YY refer to claimnumbers) is intended to provide a multiple dependent claim in thealternative form, and in some embodiments is interchangeable with theexpression “as in any one of claims XX-YY.”

When a group of substituents is disclosed herein, it is understood thatall individual members of that group and all subgroups, are disclosedseparately. When a Markush group or other grouping is used herein, allindividual members of the group and all combinations and subcombinationspossible of the group are intended to be individually included in thedisclosure. For example, when a device is set forth disclosing a rangeof materials, device components, and/or device configurations, thedescription is intended to include specific reference of eachcombination and/or variation corresponding to the disclosed range.

Every combination of components described or exemplified herein can beused to practice the invention, unless otherwise stated.

Whenever a range is given in the specification, for example, atemperature range, a composition, concentration range, or any range ofelectrochemical performance parameters (e.g., capacity, specificactivity, discharge rate, cycling, etc.) all intermediate ranges andsubranges, as well as all individual values included in the ranges givenare intended to be included in the disclosure. It will be understoodthat any subranges or individual values in a range or subrange that areincluded in the description herein can be excluded from the claimsherein.

Various embodiments and examples list materials and compounds includingvarious “oxide” and “hydroxide” compounds. Such recitations are intendedto include all variants of such materials, including oxyhydroxides,sub-oxides, solid-solutions, and various polymorphs (differentcrystal-structure forms) of the same or similar chemical compounds.

All patents and publications mentioned in the specification areindicative of the levels of skill of those skilled in the art to whichthe invention pertains. References cited herein or included as anappendix indicate the state of the art as of their publication or filingdate and it is intended that this information can be employed herein, ifneeded, to exclude specific embodiments that are in the prior art. Forexample, when composition of matter are claimed, it should be understoodthat compounds known and available in the art prior to Applicant’sinvention, including compounds for which an enabling disclosure isprovided in the references cited herein, are not intended to be includedin the composition of matter claims herein.

As used herein, “comprising” is synonymous with “including,”“containing,” or “characterized by,” and is inclusive or open-ended anddoes not exclude additional, unrecited elements or method steps. As usedherein, “consisting of” excludes any element, step, or ingredient notspecified in the claim element. As used herein, “consisting essentiallyof” does not exclude materials or steps that do not materially affectthe basic and novel characteristics of the claim. In each instanceherein any of the terms “comprising”, “consisting essentially of” and“consisting of” may be replaced with either of the other two terms. Theinvention illustratively described herein suitably may be practiced inthe absence of any element or elements, limitation or limitations whichis not specifically disclosed herein.

One of ordinary skill in the art will appreciate that electronicmaterials, fabrication processes, device components and deviceconfigurations other than those specifically exemplified can be employedin the practice of the invention without resort to undueexperimentation. All art-known functional equivalents, of any suchmaterials and methods are intended to be included in this invention. Theterms and expressions which have been employed are used as terms ofdescription and not of limitation, and there is no intention that in theuse of such terms and expressions of excluding any equivalents of thefeatures shown and described or portions thereof, but it is recognizedthat various modifications are possible within the scope of theinvention claimed. Thus, it should be understood that although thepresent invention has been specifically disclosed by preferredembodiments and optional features, modification and variation of theconcepts herein disclosed may be resorted to by those skilled in theart, and that such modifications and variations are considered to bewithin the scope of this invention as defined by the appended claims.

1. A rechargeable battery comprising: a battery container sealed againstrelease of gas up to at least a threshold gas pressure; a volume of anaqueous alkaline electrolyte at least partially filling the batterycontainer to an electrolyte level; a positive electrode containingpositive active material and at least partially submerged in the aqueousalkaline electrolyte; an iron negative electrode at least partiallysubmerged in the aqueous alkaline electrolyte, the iron negativeelectrode comprising iron active material; a separator at leastpartially submerged in the aqueous alkaline electrolyte provided betweenthe positive electrode and the iron negative electrode; and an auxiliaryoxygen gas recombination electrode electrically coupled to the ironnegative electrode, the auxiliary oxygen gas recombination electrodeionically coupled to the aqueous alkaline electrolyte, and the auxiliaryoxygen gas recombination electrode exposed to a gas headspace in thebattery container above the aqueous alkaline electrolyte a gas pathway.2-121. (canceled)
 122. The rechargeable battery of claim 1, wherein theauxiliary oxygen gas recombination electrode electrochemically reducesoxygen gas in the gas headspace.
 123. The rechargeable battery of claim1, wherein the auxiliary oxygen gas recombination electrode isphysically separated from the iron negative electrode.
 124. Therechargeable battery of claim 1, wherein the auxiliary oxygen gasrecombination electrode is at least partially submerged in the aqueousalkaline electrolyte and the gas pathway comprises a hydrophobicelement.
 125. The rechargeable battery of claim 1, wherein both the ironnegative electrode and the positive electrode contain a hydrophobicpolymer binder.
 126. The rechargeable battery of claim 125, wherein thehydrophobic polymer binder is polytetrafluoroethylene (PTFE).
 127. Therechargeable battery of claim 1, wherein the iron negative electrodeextends into the gas headspace above the electrolyte level and thepositive electrode is entirely submerged in the aqueous alkalineelectrolyte.
 128. The rechargeable battery of claim 1, wherein both thepositive electrode and the iron negative electrode extend into the gasheadspace above the electrolyte level.
 129. The rechargeable battery ofclaim 1, wherein the auxiliary oxygen gas recombination electrode isentirely submerged in the aqueous alkaline electrolyte and the gaspathway is a hydrophobic element or a PTFE element.
 130. Therechargeable battery of claim 1, wherein the auxiliary oxygen gasrecombination electrode is also electrically connected to the positiveelectrode.
 131. The rechargeable battery of claim 130, furthercomprising at least one switch actuatable to open and close circuitsbetween the auxiliary oxygen gas recombination electrode and thepositive electrode, and between the auxiliary oxygen gas recombinationelectrode and the iron negative electrode.
 132. A rechargeable batterycomprising: a battery container sealable against release of gas; apositive electrode containing positive active material; an iron negativeelectrode; a separator positionable between the positive electrode andthe iron negative electrode with the positive electrode, the ironnegative electrode, and the separator each at least partiallysubmersible in an electrolyte in the battery container; and an auxiliaryoxygen gas recombination electrode electrically coupled to the ironnegative electrode and ionically couplable to the electrolyte in thebattery container, and the auxiliary oxygen gas recombination electrodeexposable to a gas headspace in the battery container.
 133. Therechargeable battery of claim 132, wherein the auxiliary oxygen gasrecombination electrode is physically separated from the iron negativeelectrode.